Convert.ChangeType Convert.ChangeType Convert.ChangeType Convert.ChangeType Method

定义

返回一个指定类型的对象,该对象的值等效于指定的对象。Returns an object of a specified type whose value is equivalent to a specified object.

重载

ChangeType(Object, Type) ChangeType(Object, Type) ChangeType(Object, Type) ChangeType(Object, Type)

返回一个指定类型的对象,该对象的值等效于指定的对象。Returns an object of the specified type and whose value is equivalent to the specified object.

ChangeType(Object, TypeCode) ChangeType(Object, TypeCode) ChangeType(Object, TypeCode)

返回指定类型的对象,其值等效于指定对象。Returns an object of the specified type whose value is equivalent to the specified object.

ChangeType(Object, Type, IFormatProvider) ChangeType(Object, Type, IFormatProvider) ChangeType(Object, Type, IFormatProvider) ChangeType(Object, Type, IFormatProvider)

返回指定类型的对象,其值等效于指定对象。Returns an object of the specified type whose value is equivalent to the specified object. 参数提供区域性特定的格式设置信息。A parameter supplies culture-specific formatting information.

ChangeType(Object, TypeCode, IFormatProvider) ChangeType(Object, TypeCode, IFormatProvider) ChangeType(Object, TypeCode, IFormatProvider)

返回指定类型的对象,其值等效于指定对象。Returns an object of the specified type whose value is equivalent to the specified object. 参数提供区域性特定的格式设置信息。A parameter supplies culture-specific formatting information.

ChangeType(Object, Type) ChangeType(Object, Type) ChangeType(Object, Type) ChangeType(Object, Type)

返回一个指定类型的对象,该对象的值等效于指定的对象。Returns an object of the specified type and whose value is equivalent to the specified object.

public:
 static System::Object ^ ChangeType(System::Object ^ value, Type ^ conversionType);
public static object ChangeType (object value, Type conversionType);
static member ChangeType : obj * Type -> obj
Public Shared Function ChangeType (value As Object, conversionType As Type) As Object

参数

value
Object Object Object Object

一个实现 IConvertible 接口的对象。An object that implements the IConvertible interface.

conversionType
Type Type Type Type

要返回的对象的类型。The type of object to return.

返回

一个对象,其类型为 conversionType,并且其值等效于 valueAn object whose type is conversionType and whose value is equivalent to value.

- 或 --or- 如果 Nothingvalue,并且 null 不是值类型,则为空引用(在 Visual Basic 中为 conversionType)。A null reference (Nothing in Visual Basic), if value is null and conversionType is not a value type.

异常

不支持此转换。This conversion is not supported.

-or- valuenull 并且 conversionType 是一个值类型。value is null and conversionType is a value type.

- 或 --or- value 不实现 IConvertible 接口。value does not implement the IConvertible interface.

value 的格式不是 conversionType 可识别的格式。value is not in a format recognized by conversionType.

value 表示不在 conversionType 的范围内的数字。value represents a number that is out of the range of conversionType.

示例

下面的示例演示 ChangeType 方法的用法。The following example illustrates the use of the ChangeType method.

using namespace System;

int main()
{
   Double d = -2.345;
   int i =  *safe_cast<Int32^>(Convert::ChangeType( d, int::typeid ));
   Console::WriteLine( "The double value {0} when converted to an int becomes {1}", d, i );
   String^ s = "12/12/98";
   DateTime dt =  *safe_cast<DateTime^>(Convert::ChangeType( s, DateTime::typeid ));
   Console::WriteLine( "The string value {0} when converted to a Date becomes {1}", s, dt );
}
using System;

public class ChangeTypeTest {
    public static void Main() {

        Double d = -2.345;
        int i = (int)Convert.ChangeType(d, typeof(int));

        Console.WriteLine("The double value {0} when converted to an int becomes {1}", d, i);

        string s = "12/12/98";
        DateTime dt = (DateTime)Convert.ChangeType(s, typeof(DateTime));

        Console.WriteLine("The string value {0} when converted to a Date becomes {1}", s, dt);        
    }
}
Public Class ChangeTypeTest
    
    Public Shared Sub Main()
        Dim d As [Double] = - 2.345
        Dim i As Integer = CInt(Convert.ChangeType(d, GetType(Integer)))
        
        Console.WriteLine("The double value {0} when converted to an int becomes {1}", d, i)
        Dim s As String = "12/12/98"
        Dim dt As DateTime = CType(Convert.ChangeType(s, GetType(DateTime)), DateTime)
        
        Console.WriteLine("The string value {0} when converted to a Date becomes {1}", s, dt)
    End Sub
End Class

注解

ChangeType是一个通用转换方法,它将指定value的对象转换为。 conversionTypeChangeType is a general-purpose conversion method that converts the object specified by value to conversionType. 参数可以是任何类型的对象conversionType ,也可以是Type表示任何基类型或自定义类型的对象。 valueThe value parameter can be an object of any type, and conversionType can also be a Type object that represents any base or custom type. 为了成功转换, value必须IConvertible实现接口,因为方法只是包装对适当IConvertible方法的调用。For the conversion to succeed, value must implement the IConvertible interface, because the method simply wraps a call to an appropriate IConvertible method. 方法要求支持转换value为。 conversionTypeThe method requires that conversion of value to conversionType be supported.

此方法使用当前线程的区域性进行转换。This method uses the current thread's culture for the conversion.

调用方说明

ChangeType(Object, Type)方法可以将枚举值转换为另一种类型。The ChangeType(Object, Type) method can convert an enumeration value to another type. 但是,它不能将另一种类型转换为枚举值,即使源类型是枚举的基础类型。However, it cannot convert another type to an enumeration value, even if the source type is the underlying type of the enumeration. 若要将某一类型转换为枚举值,请使用强制转换C#运算符(在中)或转换函数(在 Visual Basic 中)。To convert a type to an enumeration value, use a casting operator (in C#) or a conversion function (in Visual Basic). 下面的示例演示了与大陆枚举值之间的转换。The following example illustrates the conversion to and from a Continent enumeration value.

[!code-csharpSystem.Convert.ChangeType#5] [!code-vbSystem.Convert.ChangeType#5][!code-csharpSystem.Convert.ChangeType#5] [!code-vbSystem.Convert.ChangeType#5] ChangeType(Object, Type)方法可以将可为 null 的类型转换为另一种类型。The ChangeType(Object, Type) method can convert a nullable type to another type. 但是,它不能将另一种类型转换为可以为 null 的类型conversionType的值,即使是的Nullable<T>基础类型。若要执行转换,可以使用强制转换运算符(在中C#)或转换函数(在 Visual Basic 中)。However, it cannot convert another type to a value of a nullable type, even if conversionType is the underlying type of the Nullable<T>.To perform the conversion, you can use a casting operator (in C#) or a conversion function (in Visual Basic). 下面的示例演示了如何在可以为 null 的类型之间进行转换。The following example illustrates the conversion to and from a nullable type.

[!code-csharpSystem.Convert.ChangeType#7] [!code-vbSystem.Convert.ChangeType#7][!code-csharpSystem.Convert.ChangeType#7] [!code-vbSystem.Convert.ChangeType#7]

ChangeType(Object, TypeCode) ChangeType(Object, TypeCode) ChangeType(Object, TypeCode)

返回指定类型的对象,其值等效于指定对象。Returns an object of the specified type whose value is equivalent to the specified object.

public:
 static System::Object ^ ChangeType(System::Object ^ value, TypeCode typeCode);
public static object ChangeType (object value, TypeCode typeCode);
static member ChangeType : obj * TypeCode -> obj

参数

value
Object Object Object Object

一个实现 IConvertible 接口的对象。An object that implements the IConvertible interface.

typeCode
TypeCode TypeCode TypeCode TypeCode

要返回的对象的类型。The type of object to return.

返回

一个对象,其基础类型为 typeCode,并且其值等效于 valueAn object whose underlying type is typeCode and whose value is equivalent to value.

-or- 如果 Nothingvalue 并且 nulltypeCodeEmptyString,则为空引用(在 Visual Basic 中为 Object)。A null reference (Nothing in Visual Basic), if value is null and typeCode is Empty, String, or Object.

异常

不支持此转换。This conversion is not supported.

- 或 --or- valuenull,并且 typeCode 指定值类型。value is null and typeCode specifies a value type.

-or- value 不实现 IConvertible 接口。value does not implement the IConvertible interface.

value 的格式不是 typeCode 类型可识别的格式。value is not in a format recognized by the typeCode type.

value 表示不在 typeCode 类型的范围内的数字。value represents a number that is out of the range of the typeCode type.

示例

下面的示例演示如何使用ChangeType(Object, TypeCode)方法将Object更改为TypeCode参数指定的类型(如果可能)。The following example illustrates how to use the ChangeType(Object, TypeCode) method to change an Object to the type specified by the TypeCode parameter, if possible.

using namespace System;

void main()
{
   Double d = -2.345;
   int i = (int) Convert::ChangeType(d, TypeCode::Int32);

   Console::WriteLine("The Double {0} when converted to an Int32 is {1}", d, i);

   String^ s = "12/12/2009";
   DateTime dt = (DateTime)Convert::ChangeType(s, DateTime::typeid);

   Console::WriteLine("The String {0} when converted to a Date is {1}", s, dt);        
}
// The example displays the following output:
//    The Double -2.345 when converted to an Int32 is -2
//    The String 12/12/2009 when converted to a Date is 12/12/2009 12:00:00 AM
using System;

public class ChangeTypeTest {
    public static void Main() {

        Double d = -2.345;
        int i = (int)Convert.ChangeType(d, TypeCode.Int32);

        Console.WriteLine("The Double {0} when converted to an Int32 is {1}", d, i);

        string s = "12/12/2009";
        DateTime dt = (DateTime)Convert.ChangeType(s, typeof(DateTime));

        Console.WriteLine("The String {0} when converted to a Date is {1}", s, dt);        
    }
}
// The example displays the following output:
//    The Double -2.345 when converted to an Int32 is -2
//    The String 12/12/2009 when converted to a Date is 12/12/2009 12:00:00 AM
Public Class ChangeTypeTest
    
    Public Shared Sub Main()
        Dim d As [Double] = - 2.345
        Dim i As Integer = CInt(Convert.ChangeType(d, TypeCode.Int32))
        
        Console.WriteLine("The Double {0} when converted to an Int32 is {1}", d, i)
        Dim s As String = "12/12/2009"
        Dim dt As DateTime = CDate(Convert.ChangeType(s, TypeCode.DateTime))
        
        Console.WriteLine("The String {0} when converted to a Date is {1}", s, dt)
    End Sub 
End Class 
' The example displays the following output:
'    The Double -2.345 when converted to an Int32 is -2
'    The String 12/12/2009 when converted to a Date is 12/12/2009 12:00:00 AM

注解

ChangeType(Object, TypeCode)是一个通用转换方法,它将指定value的对象转换为typeCode指定的预定义类型。ChangeType(Object, TypeCode) is a general-purpose conversion method that converts the object specified by value to a predefined type specified by typeCode. value参数可以是任何类型的对象。The value parameter can be an object of any type. 为了成功转换, value必须IConvertible实现接口,因为方法只是包装对适当IConvertible方法的调用。For the conversion to succeed, value must implement the IConvertible interface, because the method simply wraps a call to an appropriate IConvertible method. 方法还要求支持转换value为。 typeCodeThe method also requires that conversion of value to typeCode be supported.

方法不支持将value转换为自定义类型。 ChangeType(Object, TypeCode)The ChangeType(Object, TypeCode) method does not support the conversion of value to a custom type. 若要执行此类转换,请ChangeType(Object, Type)调用方法。To perform such a conversion, call the ChangeType(Object, Type) method.

ChangeType(Object, Type, IFormatProvider) ChangeType(Object, Type, IFormatProvider) ChangeType(Object, Type, IFormatProvider) ChangeType(Object, Type, IFormatProvider)

返回指定类型的对象,其值等效于指定对象。Returns an object of the specified type whose value is equivalent to the specified object. 参数提供区域性特定的格式设置信息。A parameter supplies culture-specific formatting information.

public:
 static System::Object ^ ChangeType(System::Object ^ value, Type ^ conversionType, IFormatProvider ^ provider);
public static object ChangeType (object value, Type conversionType, IFormatProvider provider);
static member ChangeType : obj * Type * IFormatProvider -> obj
Public Shared Function ChangeType (value As Object, conversionType As Type, provider As IFormatProvider) As Object

参数

value
Object Object Object Object

一个实现 IConvertible 接口的对象。An object that implements the IConvertible interface.

conversionType
Type Type Type Type

要返回的对象的类型。The type of object to return.

provider
IFormatProvider IFormatProvider IFormatProvider IFormatProvider

一个提供区域性特定的格式设置信息的对象。An object that supplies culture-specific formatting information.

返回

一个对象,其类型为 conversionType,并且其值等效于 valueAn object whose type is conversionType and whose value is equivalent to value.

- 或 --or- value,前提是 TypevalueconversionType 相等。value, if the Type of value and conversionType are equal.

-or- 如果 Nothingvalue,并且 null 不是值类型,则为空引用(在 Visual Basic 中为 conversionType)。A null reference (Nothing in Visual Basic), if value is null and conversionType is not a value type.

异常

不支持此转换。This conversion is not supported.

-or- valuenull 并且 conversionType 是一个值类型。value is null and conversionType is a value type.

-or- value 不实现 IConvertible 接口。value does not implement the IConvertible interface.

value 的格式不是 provider 可识别的 conversionType 的格式。value is not in a format for conversionType recognized by provider.

value 表示不在 conversionType的范围内的数字。value represents a number that is out of the range of conversionType.

示例

下面的示例定义一个实现 Temperature 接口的 IConvertible 类。The following example defines a Temperature class that implements the IConvertible interface.

using namespace System;
using namespace System::Globalization;

public ref class Temperature : IConvertible
{
private:
   Decimal m_Temp;

public:
   Temperature(Decimal temperature)
   {
      m_Temp = temperature;
   }
   
   property Decimal Celsius {
      Decimal get() { return m_Temp; }
   }
   
   property Decimal Kelvin {
      Decimal get() { return m_Temp + (Decimal) 273.15; }
   }
   
   property Decimal Fahrenheit {
      Decimal get() { return Math::Round((Decimal) (m_Temp * 9 / 5 + 32), 2); }
   }
   
   virtual String^ ToString()
   override {
      return m_Temp.ToString("N2") + "�C";
   }

   // IConvertible implementations.
   virtual TypeCode GetTypeCode()
   {
      return TypeCode::Object;
   }
   
   virtual bool ToBoolean(IFormatProvider^ provider) 
   {
      if (m_Temp == 0)
         return false;
      else
         return true;
   } 
   
   virtual Byte ToByte(IFormatProvider^ provider)
   {
      if (m_Temp < Byte::MinValue || m_Temp > Byte::MaxValue)
         throw gcnew OverflowException(String::Format("{0} is out of range of the Byte type.", 
                                                   m_Temp));
      else
         return Decimal::ToByte(m_Temp);
   }
   
   virtual Char ToChar(IFormatProvider^ provider)
   {
      throw gcnew InvalidCastException("Temperature to Char conversion is not supported.");
   } 
   
   virtual DateTime ToDateTime(IFormatProvider^ provider)
   {
      throw gcnew InvalidCastException("Temperature to DateTime conversion is not supported.");
   }
   
   virtual Decimal ToDecimal(IFormatProvider^ provider)
   {
      return m_Temp;
   }
   
   virtual Double ToDouble(IFormatProvider^ provider)
   {
      return Decimal::ToDouble(m_Temp);
   }   
   
   virtual Int16 ToInt16(IFormatProvider^ provider)
   {
      if (m_Temp < Int16::MinValue || m_Temp > Int16::MaxValue)
         throw gcnew OverflowException(String::Format("{0} is out of range of the Int16 type.",
                                                   m_Temp));
      else
         return Decimal::ToInt16(m_Temp);
   }
   
   virtual Int32 ToInt32(IFormatProvider^ provider)
      {
      if (m_Temp < Int32::MinValue || m_Temp > Int32::MaxValue)
         throw gcnew OverflowException(String::Format("{0} is out of range of the Int32 type.",
                                                   m_Temp));
      else
         return Decimal::ToInt32(m_Temp);
   }
   
   virtual Int64 ToInt64(IFormatProvider^ provider)
   {
      if (m_Temp < Int64::MinValue || m_Temp > Int64::MaxValue)
         throw gcnew OverflowException(String::Format("{0} is out of range of the Int64 type.",
                                                   m_Temp));
      else
         return Decimal::ToInt64(m_Temp);
   }
   
   virtual SByte ToSByte(IFormatProvider^ provider)
   {
      if (m_Temp < SByte::MinValue || m_Temp > SByte::MaxValue)
         throw gcnew OverflowException(String::Format("{0} is out of range of the SByte type.",
                                                   m_Temp));
      else
         return Decimal::ToSByte(m_Temp);
   }

   virtual Single ToSingle(IFormatProvider^ provider)
   {
      return Decimal::ToSingle(m_Temp);
   }

   virtual String^ ToString(IFormatProvider^ provider)
   {
      return m_Temp.ToString("N2", provider) + "�C";
   }
   
   virtual Object^ ToType(Type^ conversionType, IFormatProvider^ provider)
   {
      switch (Type::GetTypeCode(conversionType))
      {
      case TypeCode::Boolean: 
            return ToBoolean(nullptr);
      case TypeCode::Byte:
            return ToByte(nullptr);
      case TypeCode::Char:
            return ToChar(nullptr);
      case TypeCode::DateTime:
            return ToDateTime(nullptr);
      case TypeCode::Decimal:
            return ToDecimal(nullptr);
      case TypeCode::Double:
            return ToDouble(nullptr);
      case TypeCode::Int16:
            return ToInt16(nullptr);
      case TypeCode::Int32:
            return ToInt32(nullptr);
      case TypeCode::Int64:
            return ToInt64(nullptr);
      case TypeCode::Object:
            if (Temperature::typeid->Equals(conversionType))
               return this;
            else
               throw gcnew InvalidCastException(String::Format("Conversion to a {0} is not supported.",
                                                            conversionType->Name));
      case TypeCode::SByte:
            return ToSByte(nullptr);
      case TypeCode::Single:
            return ToSingle(nullptr);
      case TypeCode::String:
            return ToString(provider);
      case TypeCode::UInt16:
            return ToUInt16(nullptr);
      case TypeCode::UInt32:
            return ToUInt32(nullptr);
      case TypeCode::UInt64:
            return ToUInt64(nullptr);   
         default:
            throw gcnew InvalidCastException(String::Format("Conversion to {0} is not supported.", conversionType->Name));   
      }
   }
   
   virtual UInt16 ToUInt16(IFormatProvider^ provider)
   {
      if (m_Temp < UInt16::MinValue || m_Temp > UInt16::MaxValue)
         throw gcnew OverflowException(String::Format("{0} is out of range of the UInt16 type.",
                                                   m_Temp));
      else
         return Decimal::ToUInt16(m_Temp);
   }

   virtual UInt32 ToUInt32(IFormatProvider^ provider)
   {
      if (m_Temp < UInt32::MinValue || m_Temp > UInt32::MaxValue)
         throw gcnew OverflowException(String::Format("{0} is out of range of the UInt32 type.",
                                                   m_Temp));
      else
         return Decimal::ToUInt32(m_Temp);
   }
   
   virtual UInt64 ToUInt64(IFormatProvider^ provider)
   {
      if (m_Temp < UInt64::MinValue || m_Temp > UInt64::MaxValue)
         throw gcnew OverflowException(String::Format("{0} is out of range of the UInt64 type.",
                                                   m_Temp));
      else
         return Decimal::ToUInt64(m_Temp);
   }
};
using System;
using System.Globalization;

public class Temperature : IConvertible
{
   private decimal m_Temp;

   public Temperature(decimal temperature)
   {
      this.m_Temp = temperature;
   }
   
   public decimal Celsius
   {
      get { return this.m_Temp; }   
   }
   
   public decimal Kelvin
   {
      get { return this.m_Temp + 273.15m; }    
   }
   
   public decimal Fahrenheit
   {
      get { return Math.Round((decimal) (this.m_Temp * 9 / 5 + 32), 2); }
   }
   
   public override string ToString()
   {
      return m_Temp.ToString("N2") + "°C";
   }

   // IConvertible implementations.
   public TypeCode GetTypeCode()
   {
      return TypeCode.Object;
   }
   
   public bool ToBoolean(IFormatProvider provider) 
   {
      if (m_Temp == 0)
         return false;
      else
         return true;
   } 
   
   public byte ToByte(IFormatProvider provider)
   {
      if (m_Temp < Byte.MinValue || m_Temp > Byte.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the Byte type.", 
                                                   this.m_Temp));
      else
         return Decimal.ToByte(this.m_Temp);
   }
   
   public char ToChar(IFormatProvider provider)
   {
      throw new InvalidCastException("Temperature to Char conversion is not supported.");
   } 
   
   public DateTime ToDateTime(IFormatProvider provider)
   {
      throw new InvalidCastException("Temperature to DateTime conversion is not supported.");
   }
   
   public decimal ToDecimal(IFormatProvider provider)
   {
      return this.m_Temp;
   }
   
   public double ToDouble(IFormatProvider provider)
   {
      return Decimal.ToDouble(this.m_Temp);
   }   
   
   public short ToInt16(IFormatProvider provider)
   {
      if (this.m_Temp < Int16.MinValue || this.m_Temp > Int16.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the Int16 type.",
                                                   this.m_Temp));
      else
         return Decimal.ToInt16(this.m_Temp);
   }
   
   public int ToInt32(IFormatProvider provider)
      {
      if (this.m_Temp < Int32.MinValue || this.m_Temp > Int32.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the Int32 type.",
                                                   this.m_Temp));
      else
         return Decimal.ToInt32(this.m_Temp);
   }
   
   public long ToInt64(IFormatProvider provider)
   {
      if (this.m_Temp < Int64.MinValue || this.m_Temp > Int64.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the Int64 type.",
                                                   this.m_Temp));
      else
         return Decimal.ToInt64(this.m_Temp);
   }
   
   public sbyte ToSByte(IFormatProvider provider)
   {
      if (this.m_Temp < SByte.MinValue || this.m_Temp > SByte.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the SByte type.",
                                                   this.m_Temp));
      else
         return Decimal.ToSByte(this.m_Temp);
   }

   public float ToSingle(IFormatProvider provider)
   {
      return Decimal.ToSingle(this.m_Temp);
   }

   public string ToString(IFormatProvider provider)
   {
      return m_Temp.ToString("N2", provider) + "°C";
   }
   
   public object ToType(Type conversionType, IFormatProvider provider)
   {
      switch (Type.GetTypeCode(conversionType))
      {
         case TypeCode.Boolean: 
            return this.ToBoolean(null);
         case TypeCode.Byte:
            return this.ToByte(null);
         case TypeCode.Char:
            return this.ToChar(null);
         case TypeCode.DateTime:
            return this.ToDateTime(null);
         case TypeCode.Decimal:
            return this.ToDecimal(null);
         case TypeCode.Double:
            return this.ToDouble(null);
         case TypeCode.Int16:
            return this.ToInt16(null);
         case TypeCode.Int32:
            return this.ToInt32(null);
         case TypeCode.Int64:
            return this.ToInt64(null);
         case TypeCode.Object:
            if (typeof(Temperature).Equals(conversionType))
               return this;
            else
               throw new InvalidCastException(String.Format("Conversion to a {0} is not supported.",
                                                            conversionType.Name));
         case TypeCode.SByte:
            return this.ToSByte(null);
         case TypeCode.Single:
            return this.ToSingle(null);
         case TypeCode.String:
            return this.ToString(provider);
         case TypeCode.UInt16:
            return this.ToUInt16(null);
         case TypeCode.UInt32:
            return this.ToUInt32(null);
         case TypeCode.UInt64:
            return this.ToUInt64(null);   
         default:
            throw new InvalidCastException(String.Format("Conversion to {0} is not supported.", conversionType.Name));   
      }
   }
   
   public ushort ToUInt16(IFormatProvider provider)
   {
      if (this.m_Temp < UInt16.MinValue || this.m_Temp > UInt16.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the UInt16 type.",
                                                   this.m_Temp));
      else
         return Decimal.ToUInt16(this.m_Temp);
   }

   public uint ToUInt32(IFormatProvider provider)
   {
      if (this.m_Temp < UInt32.MinValue || this.m_Temp > UInt32.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the UInt32 type.",
                                                   this.m_Temp));
      else
         return Decimal.ToUInt32(this.m_Temp);
   }
   
   public ulong ToUInt64(IFormatProvider provider)
   {
      if (this.m_Temp < UInt64.MinValue || this.m_Temp > UInt64.MaxValue)
         throw new OverflowException(String.Format("{0} is out of range of the UInt64 type.",
                                                   this.m_Temp));
      else
         return Decimal.ToUInt64(this.m_Temp);
   }
}
Imports System.Globalization

Public Class Temperature : Implements IConvertible
   Private m_Temp As Decimal

   Public Sub New(temperature As Decimal)
      Me.m_Temp = temperature
   End Sub
   
   Public ReadOnly Property Celsius() As Decimal
      Get
         Return Me.m_Temp
      End Get   
   End Property
   
   Public ReadOnly Property Kelvin() As Decimal
      Get
         Return Me.m_Temp + 273.15d   
      End Get
   End Property
   
   Public ReadOnly Property Fahrenheit() As Decimal
      Get
         Return Math.Round(CDec(Me.m_Temp * 9 / 5 + 32), 2)
      End Get      
   End Property
   
   Public Overrides Function ToString() As String
      Return m_Temp.ToString("N2") & "°C"
   End Function

   ' IConvertible implementations.
   Public Function GetTypeCode() As TypeCode _
                   Implements IConvertible.GetTypeCode
      Return TypeCode.Object
   End Function
   
   Public Function ToBoolean(provider As IFormatProvider) As Boolean _
                   Implements IConvertible.ToBoolean
      If m_Temp = 0 Then
         Return False
      Else
         Return True
      End If
   End Function 
   
   Public Function ToByte(provider As IFormatProvider) As Byte _
                   Implements IConvertible.ToByte
      If m_Temp < Byte.MinValue Or m_Temp > Byte.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the Byte type.", _ 
                                                   Me.m_Temp)) 
      Else
         Return Decimal.ToByte(Me.m_Temp)
      End If       
   End Function
   
   Public Function ToChar(provider As IFormatProvider) As Char _
                   Implements IConvertible.ToChar
      Throw New InvalidCastException("Temperature to Char conversion is not supported.")
   End Function 
   
   Public Function ToDateTime(provider As IFormatProvider) As Date _
                   Implements IConvertible.ToDateTime
      Throw New InvalidCastException("Temperature to DateTime conversion is not supported.")
   End Function
   
   Public Function ToDecimal(provider As IFormatProvider) As Decimal _
                   Implements IConvertible.ToDecimal
      Return Me.m_Temp
   End Function
   
   Public Function ToDouble(provider As IFormatProvider) As Double _
                   Implements IConvertible.ToDouble
      Return Decimal.ToDouble(Me.m_Temp)
   End Function   
   
   Public Function ToInt16(provider As IFormatProvider) As Int16 _
                   Implements IConvertible.ToInt16
      If Me.m_Temp < Int16.MinValue Or Me.m_Temp > Int16.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the Int16 type.", _
                                                   Me.m_Temp))
      Else
         Return Decimal.ToInt16(Me.m_Temp)   
      End If
   End Function
   
   Public Function ToInt32(provider As IFormatProvider) As Int32 _
                   Implements IConvertible.ToInt32
      If Me.m_Temp < Int32.MinValue Or Me.m_Temp > Int32.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the Int32 type.", _
                                                   Me.m_Temp))
      Else
         Return Decimal.ToInt32(Me.m_Temp)
      End If      
   End Function
   
   Public Function ToInt64(provider As IFormatProvider) As Int64 _
                   Implements IConvertible.ToInt64
      If Me.m_Temp < Int64.MinValue Or Me.m_Temp > Int64.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the Int64 type.", _
                                                   Me.m_Temp))
      Else
         Return Decimal.ToInt64(Me.m_Temp)
      End If      
   End Function
   
   Public Function ToSByte(provider As IFormatProvider) As SByte _
                   Implements IConvertible.ToSByte
      If Me.m_Temp < SByte.MinValue Or Me.m_Temp > SByte.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the SByte type.", _
                                                   Me.m_Temp))
      Else
         Return Decimal.ToSByte(Me.m_Temp)
      End If      
   End Function

   Public Function ToSingle(provider As IFormatProvider) As Single _
                   Implements IConvertible.ToSingle
      Return Decimal.ToSingle(Me.m_Temp)
   End Function

   Public Overloads Function ToString(provider As IFormatProvider) As String _
                   Implements IConvertible.ToString
      Return m_Temp.ToString("N2", provider) & "°C"
   End Function
   
   Public Function ToType(conversionType As Type, provider As IFormatProvider) As Object _
                   Implements IConvertible.ToType
      Select Case Type.GetTypeCode(conversionType)
         Case TypeCode.Boolean 
            Return Me.ToBoolean(Nothing)
         Case TypeCode.Byte
            Return Me.ToByte(Nothing)
         Case TypeCode.Char
            Return Me.ToChar(Nothing)
         Case TypeCode.DateTime
            Return Me.ToDateTime(Nothing)
         Case TypeCode.Decimal
            Return Me.ToDecimal(Nothing)
         Case TypeCode.Double
            Return Me.ToDouble(Nothing)
         Case TypeCode.Int16
            Return Me.ToInt16(Nothing)
         Case TypeCode.Int32
            Return Me.ToInt32(Nothing)
         Case TypeCode.Int64
            Return Me.ToInt64(Nothing)
         Case TypeCode.Object
            If GetType(Temperature).Equals(conversionType) Then
               Return Me
            Else
               Throw New InvalidCastException(String.Format("Conversion to a {0} is not supported.", _
                                                            conversionType.Name))
            End If 
         Case TypeCode.SByte
            Return Me.ToSByte(Nothing)
         Case TypeCode.Single
            Return Me.ToSingle(Nothing)
         Case TypeCode.String
            Return Me.ToString(provider)
         Case TypeCode.UInt16
            Return Me.ToUInt16(Nothing)
         Case TypeCode.UInt32
            Return Me.ToUInt32(Nothing)
         Case TypeCode.UInt64
            Return Me.ToUInt64(Nothing)   
         Case Else
            Throw New InvalidCastException(String.Format("Conversion to {0} is not supported.", conversionType.Name))   
      End Select
   End Function
   
   Public Function ToUInt16(provider As IFormatProvider) As UInt16 _
                   Implements IConvertible.ToUInt16
      If Me.m_Temp < UInt16.MinValue Or Me.m_Temp > UInt16.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the UInt16 type.", _
                                                   Me.m_Temp))
      Else
         Return Decimal.ToUInt16(Me.m_Temp)
      End If   
   End Function

   Public Function ToUInt32(provider As IFormatProvider) As UInt32 _
                   Implements IConvertible.ToUInt32
      If Me.m_Temp < UInt32.MinValue Or Me.m_Temp > UInt32.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the UInt32 type.", _
                                                   Me.m_Temp))
      Else
         Return Decimal.ToUInt32(Me.m_Temp)
      End If   
   End Function
   
   Public Function ToUInt64(provider As IFormatProvider) As UInt64 _
                   Implements IConvertible.ToUInt64
      If Me.m_Temp < UInt64.MinValue Or Me.m_Temp > UInt64.MaxValue Then
         Throw New OverflowException(String.Format("{0} is out of range of the UInt64 type.", _
                                                   Me.m_Temp))
      Else
         Return Decimal.ToUInt64(Me.m_temp)
      End If   
   End Function
End Class

下面的示例创建Temperature类的实例并ChangeType(Object, Type, IFormatProvider)调用方法,以将其转换为 .NET Framework 和的String基本数值类型。The following example creates an instance of the Temperature class and calls the ChangeType(Object, Type, IFormatProvider) method to convert it to the basic numeric types supported by the .NET Framework and to a String. 它阐释ChangeType了方法包装对源类型的IConvertible实现的调用。It illustrates that the ChangeType method wraps a call to the source type's IConvertible implementation.

void main()
{     
   Temperature^ cool = gcnew Temperature(5);
   array<Type^>^ targetTypes = gcnew array<Type^> { SByte::typeid, Int16::typeid, Int32::typeid,
                                                    Int64::typeid, Byte::typeid, UInt16::typeid,
                                                    UInt32::typeid, UInt64::typeid, Decimal::typeid,
                                                    Single::typeid, Double::typeid, String::typeid };
   CultureInfo^ provider = gcnew CultureInfo("fr-FR");
      
   for each (Type^ targetType in targetTypes)
   {
      try {
         Object^ value = Convert::ChangeType(cool, targetType, provider);
         Console::WriteLine("Converted {0} {1} to {2} {3}.",
                           cool->GetType()->Name, cool->ToString(),
                           targetType->Name, value);
      }
      catch (InvalidCastException^) {
         Console::WriteLine("Unsupported {0} --> {1} conversion.",
                           cool->GetType()->Name, targetType->Name);
      }                     
      catch (OverflowException^) {
         Console::WriteLine("{0} is out of range of the {1} type.",
                           cool, targetType->Name);
      }
   }
}
// The example dosplays the following output:
//       Converted Temperature 5.00�C to SByte 5.
//       Converted Temperature 5.00�C to Int16 5.
//       Converted Temperature 5.00�C to Int32 5.
//       Converted Temperature 5.00�C to Int64 5.
//       Converted Temperature 5.00�C to Byte 5.
//       Converted Temperature 5.00�C to UInt16 5.
//       Converted Temperature 5.00�C to UInt32 5.
//       Converted Temperature 5.00�C to UInt64 5.
//       Converted Temperature 5.00�C to Decimal 5.
//       Converted Temperature 5.00�C to Single 5.
//       Converted Temperature 5.00�C to Double 5.
//       Converted Temperature 5.00�C to String 5,00�C.
public class Example
{
   public static void Main()
   {
      Temperature cool = new Temperature(5);
      Type[] targetTypes = { typeof(SByte), typeof(Int16), typeof(Int32),
                             typeof(Int64), typeof(Byte), typeof(UInt16),
                             typeof(UInt32), typeof(UInt64), typeof(Decimal),
                             typeof(Single), typeof(Double), typeof(String) };
      CultureInfo provider = new CultureInfo("fr-FR");
      
      foreach (Type targetType in targetTypes)
      {
         try {
            object value = Convert.ChangeType(cool, targetType, provider);
            Console.WriteLine("Converted {0} {1} to {2} {3}.",
                              cool.GetType().Name, cool.ToString(),
                              targetType.Name, value);
         }
         catch (InvalidCastException) {
            Console.WriteLine("Unsupported {0} --> {1} conversion.",
                              cool.GetType().Name, targetType.Name);
         }                     
         catch (OverflowException) {
            Console.WriteLine("{0} is out of range of the {1} type.",
                              cool, targetType.Name);
         }
      }
   }
}
// The example dosplays the following output:
//       Converted Temperature 5.00°C to SByte 5.
//       Converted Temperature 5.00°C to Int16 5.
//       Converted Temperature 5.00°C to Int32 5.
//       Converted Temperature 5.00°C to Int64 5.
//       Converted Temperature 5.00°C to Byte 5.
//       Converted Temperature 5.00°C to UInt16 5.
//       Converted Temperature 5.00°C to UInt32 5.
//       Converted Temperature 5.00°C to UInt64 5.
//       Converted Temperature 5.00°C to Decimal 5.
//       Converted Temperature 5.00°C to Single 5.
//       Converted Temperature 5.00°C to Double 5.
//       Converted Temperature 5.00°C to String 5,00°C.
Module Example
   Public Sub Main()
      Dim cool As New Temperature(5)
      Dim targetTypes() As Type = { GetType(SByte), GetType(Int16), GetType(Int32), _
                                    GetType(Int64), GetType(Byte), GetType(UInt16), _
                                    GetType(UInt32), GetType(UInt64), GetType(Decimal), _
                                    GetType(Single), GetType(Double), GetType(String) }
      Dim provider As New CultureInfo("fr-FR")
      
      For Each targetType As Type In targetTypes
         Try
            Dim value As Object = Convert.ChangeType(cool, targetType, provider)
            Console.WriteLine("Converted {0} {1} to {2} {3}.", _
                              cool.GetType().Name, cool.ToString(), _
                              targetType.Name, value)
         Catch e As InvalidCastException
            Console.WriteLine("Unsupported {0} --> {1} conversion.", _
                              cool.GetType().Name, targetType.Name)
         Catch e As OverflowException
            Console.WriteLine("{0} is out of range of the {1} type.", _
                              cool, targetType.Name)
         End Try                     
      Next
   End Sub
End Module
' The example displays the following output:
'       Converted Temperature 5.00°C to SByte 5.
'       Converted Temperature 5.00°C to Int16 5.
'       Converted Temperature 5.00°C to Int32 5.
'       Converted Temperature 5.00°C to Int64 5.
'       Converted Temperature 5.00°C to Byte 5.
'       Converted Temperature 5.00°C to UInt16 5.
'       Converted Temperature 5.00°C to UInt32 5.
'       Converted Temperature 5.00°C to UInt64 5.
'       Converted Temperature 5.00°C to Decimal 5.
'       Converted Temperature 5.00°C to Single 5.
'       Converted Temperature 5.00°C to Double 5.
'       Converted Temperature 5.00°C to String 5,00°C.

注解

ChangeType是一个通用转换方法,它将指定value的对象转换为。 conversionTypeChangeType is a general-purpose conversion method that converts the object specified by value to conversionType. 参数可以是任何类型的对象conversionType ,也可以是Type表示任何基类型或自定义类型的对象。 valueThe value parameter can be an object of any type, and conversionType can also be a Type object that represents any base or custom type. 为了成功转换, value必须IConvertible实现接口,因为方法只是包装对适当IConvertible方法的调用。For the conversion to succeed, value must implement the IConvertible interface, because the method simply wraps a call to an appropriate IConvertible method. 方法要求支持转换value为。 conversionTypeThe method requires that conversion of value to conversionType be supported.

参数是为转换提供格式设置信息的实现。IFormatProvider providerThe provider parameter is an IFormatProvider implementation that supplies formatting information for the conversion. 是否以及如何使用此参数取决于基础IConvertible实现。Whether and how this parameter is used depends on the underlying IConvertible implementation. 如果value是基本数据类型, provider则仅用于以下转换:If value is a base data type, provider is used only for the following conversions:

如果value是应用程序定义的类型,则IConvertible其实现可能使用provider参数。If value is an application-defined type, its IConvertible implementation may use the provider parameter.

调用方说明

ChangeType(Object, Type, IFormatProvider)方法可以将枚举值转换为另一种类型。The ChangeType(Object, Type, IFormatProvider) method can convert an enumeration value to another type. 但是,它不能将另一种类型转换为枚举值,即使源类型是枚举的基础类型。However, it cannot convert another type to an enumeration value, even if the source type is the underlying type of the enumeration. 若要将某一类型转换为枚举值,请使用强制转换C#运算符(在中)或转换函数(在 Visual Basic 中)。To convert a type to an enumeration value, use a casting operator (in C#) or a conversion function (in Visual Basic). 下面的示例演示了与大陆枚举值之间的转换。The following example illustrates the conversion to and from a Continent enumeration value.

[!code-csharpSystem.Convert.ChangeType#5] [!code-vbSystem.Convert.ChangeType#5][!code-csharpSystem.Convert.ChangeType#5] [!code-vbSystem.Convert.ChangeType#5] ChangeType(Object, Type, IFormatProvider)方法可以将可为 null 的类型转换为另一种类型。The ChangeType(Object, Type, IFormatProvider) method can convert a nullable type to another type. 但是,它不能将另一种类型转换为可以为 null 的类型conversionType的值,即使是的Nullable<T>基础类型。However, it cannot convert another type to a value of a nullable type, even if conversionType is the underlying type of the Nullable<T>. 若要执行转换,可以使用强制转换运算符(在中C#)或转换函数(在 Visual Basic 中)。To perform the conversion, you can use a casting operator (in C#) or a conversion function (in Visual Basic). 下面的示例演示了如何在可以为 null 的类型之间进行转换。The following example illustrates the conversion to and from a nullable type.

[!code-csharpSystem.Convert.ChangeType#8] [!code-vbSystem.Convert.ChangeType#8][!code-csharpSystem.Convert.ChangeType#8] [!code-vbSystem.Convert.ChangeType#8]

ChangeType(Object, TypeCode, IFormatProvider) ChangeType(Object, TypeCode, IFormatProvider) ChangeType(Object, TypeCode, IFormatProvider)

返回指定类型的对象,其值等效于指定对象。Returns an object of the specified type whose value is equivalent to the specified object. 参数提供区域性特定的格式设置信息。A parameter supplies culture-specific formatting information.

public:
 static System::Object ^ ChangeType(System::Object ^ value, TypeCode typeCode, IFormatProvider ^ provider);
public static object ChangeType (object value, TypeCode typeCode, IFormatProvider provider);
static member ChangeType : obj * TypeCode * IFormatProvider -> obj

参数

value
Object Object Object Object

一个实现 IConvertible 接口的对象。An object that implements the IConvertible interface.

typeCode
TypeCode TypeCode TypeCode TypeCode

要返回的对象的类型。The type of object to return.

provider
IFormatProvider IFormatProvider IFormatProvider IFormatProvider

一个提供区域性特定的格式设置信息的对象。An object that supplies culture-specific formatting information.

返回

一个对象,其基础类型为 typeCode,并且其值等效于 valueAn object whose underlying type is typeCode and whose value is equivalent to value.

-or- 如果 Nothingvalue 并且 nulltypeCodeEmptyString,则为空引用(在 Visual Basic 中为 Object)。A null reference (Nothing in Visual Basic), if value is null and typeCode is Empty, String, or Object.

异常

不支持此转换。This conversion is not supported.

-or- valuenull,并且 typeCode 指定值类型。value is null and typeCode specifies a value type.

-or- value 不实现 IConvertible 接口。value does not implement the IConvertible interface.

value 的格式不是 provider 可识别的 typeCode 类型的格式。value is not in a format for the typeCode type recognized by provider.

value 表示不在 typeCode 类型的范围内的数字。value represents a number that is out of the range of the typeCode type.

示例

下面的示例定义了一个InterceptProvider名为的自定义格式提供程序,该提供程序指示何时调用其GetFormat方法,并NumberFormatInfo DateTimeFormatInfo为 fr-fr 区域性和 en-us 区域性的对象返回。The following example defines a custom format provider named InterceptProvider that indicates when its GetFormat method is called and returns a NumberFormatInfo for the fr-FR culture and a DateTimeFormatInfo object for the en-US culture. 此格式提供程序用于对方法的ChangeType(Object, TypeCode, IFormatProvider)所有调用。This format provider is used in all calls to the ChangeType(Object, TypeCode, IFormatProvider) method. 然后,该Double示例创建一个具有DateTime和值的数组ChangeType(Object, TypeCode, IFormatProvider) ,并对每个值和TypeCode枚举的每个成员进行重复调用。The example then creates an array with a Double and a DateTime value and makes repeated calls to ChangeType(Object, TypeCode, IFormatProvider) with each value and each member of the TypeCode enumeration. 该示例阐释了方法何时使用IFormatProvider参数,并说明了如何使用provider参数来执行区分区域性的格式设置。The example illustrates when the method uses the IFormatProvider parameter and also illustrates the use of the provider parameter to perform culture-sensitive formatting.

using namespace System;
using namespace System::Globalization;

ref class InterceptProvider : IFormatProvider
{
public: 
   virtual Object^ GetFormat(Type^ formatType) 
   {
      CultureInfo^ culture;
      if (formatType == NumberFormatInfo::typeid) {
         Console::WriteLine("   Returning a fr-FR numeric format provider.");
         
         culture = gcnew CultureInfo("fr-FR");
         return culture->NumberFormat;
      }  
      else if (formatType == DateTimeFormatInfo::typeid) {
         Console::WriteLine("   Returning an en-US date/time format provider.");
         culture = gcnew CultureInfo("en-US");
         return culture->DateTimeFormat;
      }
      else {
         Console::WriteLine("   Requesting a format provider of {0}.", formatType->Name);
         return nullptr;
      }
   }
};

void main()
{
   array<Object^>^ values = gcnew array<Object^> { 103.5, gcnew DateTime(2010, 12, 26, 14, 34, 0) };
   IFormatProvider^ provider = gcnew InterceptProvider();
      
   // Convert value to each of the types represented in TypeCode enum.
   for each (Object^ value in values)
   {
      // Iterate types in TypeCode enum.
      for each (TypeCode enumType in (array<TypeCode>^) Enum::GetValues(TypeCode::typeid))
      {         
         if (enumType == TypeCode::DBNull || enumType == TypeCode::Empty) continue;
            
         try {
            Console::WriteLine("{0} ({1}) --> {2} ({3}).", 
                              value, value->GetType()->Name,
                              Convert::ChangeType(value, enumType, provider),
                              enumType.ToString());
         }
         catch (InvalidCastException^ e) {
            Console::WriteLine("Cannot convert a {0} to a {1}",
                              value->GetType()->Name, enumType.ToString());
         }                     
         catch (OverflowException^ e) {
            Console::WriteLine("Overflow: {0} is out of the range of a {1}",
                              value, enumType.ToString());
         }
      }
      Console::WriteLine();
   }
}
// The example displays the following output:
//    103.5 (Double) --> 103.5 (Object).
//    103.5 (Double) --> True (Boolean).
//    Cannot convert a Double to a Char
//    103.5 (Double) --> 104 (SByte).
//    103.5 (Double) --> 104 (Byte).
//    103.5 (Double) --> 104 (Int16).
//    103.5 (Double) --> 104 (UInt16).
//    103.5 (Double) --> 104 (Int32).
//    103.5 (Double) --> 104 (UInt32).
//    103.5 (Double) --> 104 (Int64).
//    103.5 (Double) --> 104 (UInt64).
//    103.5 (Double) --> 103.5 (Single).
//    103.5 (Double) --> 103.5 (Double).
//    103.5 (Double) --> 103.5 (Decimal).
//    Cannot convert a Double to a DateTime
//       Returning a fr-FR numeric format provider.
//    103.5 (Double) --> 103,5 (String).
//    
//    12/26/2010 2:34:00 PM (DateTime) --> 12/26/2010 2:34:00 PM (Object).
//    Cannot convert a DateTime to a Boolean
//    Cannot convert a DateTime to a Char
//    Cannot convert a DateTime to a SByte
//    Cannot convert a DateTime to a Byte
//    Cannot convert a DateTime to a Int16
//    Cannot convert a DateTime to a UInt16
//    Cannot convert a DateTime to a Int32
//    Cannot convert a DateTime to a UInt32
//    Cannot convert a DateTime to a Int64
//    Cannot convert a DateTime to a UInt64
//    Cannot convert a DateTime to a Single
//    Cannot convert a DateTime to a Double
//    Cannot convert a DateTime to a Decimal
//    12/26/2010 2:34:00 PM (DateTime) --> 12/26/2010 2:34:00 PM (DateTime).
//       Returning an en-US date/time format provider.
//    12/26/2010 2:34:00 PM (DateTime) --> 12/26/2010 2:34:00 PM (String).
using System;
using System.Globalization;

public class InterceptProvider : IFormatProvider
{
   public object GetFormat(Type formatType) 
   {
      if (formatType == typeof(NumberFormatInfo)) {
         Console.WriteLine("   Returning a fr-FR numeric format provider.");
         return new System.Globalization.CultureInfo("fr-FR").NumberFormat;
      }   
      else if (formatType == typeof(DateTimeFormatInfo)) {
         Console.WriteLine("   Returning an en-US date/time format provider.");
         return new System.Globalization.CultureInfo("en-US").DateTimeFormat;
      }
      else {
         Console.WriteLine("   Requesting a format provider of {0}.", formatType.Name);
         return null;
      }
   }
}

public class Example
{
   public static void Main()
   {
      object[] values = { 103.5d, new DateTime(2010, 12, 26, 14, 34, 0) };
      IFormatProvider provider = new InterceptProvider();
      
      // Convert value to each of the types represented in TypeCode enum.
      foreach (object value in values)
      {
         // Iterate types in TypeCode enum.
         foreach (TypeCode enumType in ((TypeCode[]) Enum.GetValues(typeof(TypeCode))))
         {         
            if (enumType == TypeCode.DBNull || enumType == TypeCode.Empty) continue;
            
            try {
               Console.WriteLine("{0} ({1}) --> {2} ({3}).", 
                                 value, value.GetType().Name,
                                 Convert.ChangeType(value, enumType, provider),
                                 enumType.ToString());
            }
            catch (InvalidCastException) {
               Console.WriteLine("Cannot convert a {0} to a {1}",
                                 value.GetType().Name, enumType.ToString());
            }                     
            catch (OverflowException) {
               Console.WriteLine("Overflow: {0} is out of the range of a {1}",
                                 value, enumType.ToString());
            }
         }
         Console.WriteLine();
      }
   }
}
// The example displays the following output:
//    103.5 (Double) --> 103.5 (Object).
//    103.5 (Double) --> True (Boolean).
//    Cannot convert a Double to a Char
//    103.5 (Double) --> 104 (SByte).
//    103.5 (Double) --> 104 (Byte).
//    103.5 (Double) --> 104 (Int16).
//    103.5 (Double) --> 104 (UInt16).
//    103.5 (Double) --> 104 (Int32).
//    103.5 (Double) --> 104 (UInt32).
//    103.5 (Double) --> 104 (Int64).
//    103.5 (Double) --> 104 (UInt64).
//    103.5 (Double) --> 103.5 (Single).
//    103.5 (Double) --> 103.5 (Double).
//    103.5 (Double) --> 103.5 (Decimal).
//    Cannot convert a Double to a DateTime
//       Returning a fr-FR numeric format provider.
//    103.5 (Double) --> 103,5 (String).
//    
//    12/26/2010 2:34:00 PM (DateTime) --> 12/26/2010 2:34:00 PM (Object).
//    Cannot convert a DateTime to a Boolean
//    Cannot convert a DateTime to a Char
//    Cannot convert a DateTime to a SByte
//    Cannot convert a DateTime to a Byte
//    Cannot convert a DateTime to a Int16
//    Cannot convert a DateTime to a UInt16
//    Cannot convert a DateTime to a Int32
//    Cannot convert a DateTime to a UInt32
//    Cannot convert a DateTime to a Int64
//    Cannot convert a DateTime to a UInt64
//    Cannot convert a DateTime to a Single
//    Cannot convert a DateTime to a Double
//    Cannot convert a DateTime to a Decimal
//    12/26/2010 2:34:00 PM (DateTime) --> 12/26/2010 2:34:00 PM (DateTime).
//       Returning an en-US date/time format provider.
//    12/26/2010 2:34:00 PM (DateTime) --> 12/26/2010 2:34:00 PM (String).
Imports System.Globalization

Public Class InterceptProvider : Implements IFormatProvider
   Public Function GetFormat(formatType As Type) As Object _
          Implements IFormatProvider.GetFormat
      If formatType.Equals(GetType(NumberFormatInfo)) Then
         Console.WriteLine("   Returning a fr-FR numeric format provider.")
         Return New CultureInfo("fr-FR").NumberFormat
      ElseIf formatType.Equals(GetType(DateTimeFormatInfo)) Then
         Console.WriteLine("   Returning an en-US date/time format provider.")
         Return New CultureInfo("en-US").DateTimeFormat
      Else
         Console.WriteLine("   Requesting a format provider of {0}.", formatType.Name)
         Return Nothing
      End If
   End Function
End Class

Module Example
   Public Sub Main()
      Dim values() As Object = { 103.5r, #12/26/2010 2:34PM# }
      Dim provider As New InterceptProvider()
      
      ' Convert value to each of the types represented in TypeCode enum.
      For Each value As Object In values
         ' Iterate types in TypeCode enum.
         For Each enumType As TypeCode In DirectCast([Enum].GetValues(GetType(TypeCode)), TypeCode())         
            If enumType = TypeCode.DbNull Or enumType = TypeCode.Empty Then Continue For
            
            Try
               Console.WriteLine("{0} ({1}) --> {2} ({3}).", _
                                 value, value.GetType().Name, _
                                 Convert.ChangeType(value, enumType, provider), _
                                 enumType.ToString())
            Catch e As InvalidCastException
               Console.WriteLine("Cannot convert a {0} to a {1}", _
                                 value.GetType().Name, enumType.ToString())
            Catch e As OverflowException
               Console.WriteLine("Overflow: {0} is out of the range of a {1}", _
                                 value, enumType.ToString())
            End Try
         Next
         Console.WriteLine()
      Next
   End Sub
End Module
' The example displays the following output:
'    103.5 (Double) --> 103.5 (Object).
'    103.5 (Double) --> True (Boolean).
'    Cannot convert a Double to a Char
'    103.5 (Double) --> 104 (SByte).
'    103.5 (Double) --> 104 (Byte).
'    103.5 (Double) --> 104 (Int16).
'    103.5 (Double) --> 104 (UInt16).
'    103.5 (Double) --> 104 (Int32).
'    103.5 (Double) --> 104 (UInt32).
'    103.5 (Double) --> 104 (Int64).
'    103.5 (Double) --> 104 (UInt64).
'    103.5 (Double) --> 103.5 (Single).
'    103.5 (Double) --> 103.5 (Double).
'    103.5 (Double) --> 103.5 (Decimal).
'    Cannot convert a Double to a DateTime
'       Returning a fr-FR numeric format provider.
'    103.5 (Double) --> 103,5 (String).
'    
'    12/26/2010 2:34:00 PM (DateTime) --> 12/26/2010 2:34:00 PM (Object).
'    Cannot convert a DateTime to a Boolean
'    Cannot convert a DateTime to a Char
'    Cannot convert a DateTime to a SByte
'    Cannot convert a DateTime to a Byte
'    Cannot convert a DateTime to a Int16
'    Cannot convert a DateTime to a UInt16
'    Cannot convert a DateTime to a Int32
'    Cannot convert a DateTime to a UInt32
'    Cannot convert a DateTime to a Int64
'    Cannot convert a DateTime to a UInt64
'    Cannot convert a DateTime to a Single
'    Cannot convert a DateTime to a Double
'    Cannot convert a DateTime to a Decimal
'    12/26/2010 2:34:00 PM (DateTime) --> 12/26/2010 2:34:00 PM (DateTime).
'       Returning an en-US date/time format provider.
'    12/26/2010 2:34:00 PM (DateTime) --> 12/26/2010 2:34:00 PM (String).

注解

ChangeType(Object, TypeCode, IFormatProvider)是一个通用转换方法,它将指定value的对象转换为typeCode指定的预定义类型。ChangeType(Object, TypeCode, IFormatProvider) is a general-purpose conversion method that converts the object specified by value to a predefined type specified by typeCode. value参数可以是任何类型的对象。The value parameter can be an object of any type. 为了成功转换, value必须IConvertible实现接口,因为方法只是包装对适当IConvertible方法的调用。For the conversion to succeed, value must implement the IConvertible interface, because the method simply wraps a call to an appropriate IConvertible method. 方法还要求支持转换value为。 typeCodeThe method also requires that conversion of value to typeCode be supported.

方法不支持将value转换为自定义类型。 ChangeType(Object, TypeCode, IFormatProvider)The ChangeType(Object, TypeCode, IFormatProvider) method does not support the conversion of value to a custom type. 若要执行此类转换,请ChangeType(Object, Type, IFormatProvider)调用方法。To perform such a conversion, call the ChangeType(Object, Type, IFormatProvider) method.

参数是为转换提供格式设置信息的实现。IFormatProvider providerThe provider parameter is an IFormatProvider implementation that supplies formatting information for the conversion. 是否以及如何使用此参数取决于基础IConvertible实现。Whether and how this parameter is used depends on the underlying IConvertible implementation. 如果value是基本数据类型, provider则仅用于以下转换。If value is a base data type, provider is used only for the following conversions. 如果将参数传递给这些方法,则使用表示当前线程区域性的对象。CultureInfo IFormatProvider nullIf a nullIFormatProvider argument is passed to these methods, the CultureInfo object that represents the current thread culture is used.

如果value是应用程序定义的类型,则IConvertible其实现可能使用provider参数。If value is an application-defined type, its IConvertible implementation may use the provider parameter.

适用于