Nullable<T>.HasValue Nullable<T>.HasValue Nullable<T>.HasValue Nullable<T>.HasValue Property

定义

获取一个值,该值指示 Nullable<T> 对象是否具有基础类型的有效值。Gets a value indicating whether the current Nullable<T> object has a valid value of its underlying type.

public:
 property bool HasValue { bool get(); };
public bool HasValue { get; }
member this.HasValue : bool
Public ReadOnly Property HasValue As Boolean

属性值

如果当前的 true 对象具有值,则为 Nullable<T>;如果当前的 Nullable<T> 对象没有值,则为 falsetrue if the current Nullable<T> object has a value; false if the current Nullable<T> object has no value.

示例

下面的HasValue示例使用Nullable<int>Nullable(Of Integer)位于 Visual Basic)对象的属性来确定它是否应显示对象的Value属性或其GetValueOrDefault属性。The following example uses the HasValue property of a Nullable<int> (Nullable(Of Integer) in Visual Basic) object to determine whether it should display the object's Value property or its GetValueOrDefault property.

using System;

public class Example
{
   public static void Main()
   {
      Nullable<int> n1 = new Nullable<int>(10);
      Nullable<int> n2 = null;
      Nullable<int> n3 = new Nullable<int>(20);
      n3 = null;
      Nullable<int>[] items = { n1, n2, n3 }; 

      foreach (var item in items) {
         Console.WriteLine("Has a value: {0}", item.HasValue);
         if (item.HasValue) {
            Console.WriteLine("Type: {0}", item.GetType().Name);
            Console.WriteLine("Value: {0}", item.Value);
         }
         else {
            Console.WriteLine("Null: {0}", item == null);
            Console.WriteLine("Default Value: {0}", item.GetValueOrDefault());
         }
         Console.WriteLine();
      }                  
   }
}
// The example displays the following output:
//       Has a value: True
//       Type: Int32
//       Value: 10
//       
//       Has a value: False
//       Null: True
//       Default Value: 0
//       
//       Has a value: False
//       Null: True
//       Default Value: 0
Module Example
   Public Sub Main()
      Dim n1 As New Nullable(Of Integer)(10)
      Dim n2 As Nullable(Of Integer)
      Dim n3 As New Nullable(Of Integer)(20)
      n3 = Nothing
      Dim items() As Nullable(Of Integer) = { n1, n2, n3 }

      For Each item In items
         Console.WriteLine("Has a value: {0}", item.HasValue)
         If item.HasValue Then
            Console.WriteLine("Type: {0}", item.GetType().Name)
            Console.WriteLine("Value: {0}", item.Value)
         Else
            Console.WriteLine("Null: {0}", item Is Nothing)
            Console.WriteLine("Default Value: {0}", item.GetValueOrDefault())
         End If
         Console.WriteLine()
      Next                  
   End Sub
End Module
' The example displays the following output:
'       Has a value: True
'       Type: Int32
'       Value: 10
'       
'       Has a value: False
'       Null: True
'       Default Value: 0
'       
'       Has a value: False
'       Null: True
'       Default Value: 0

注解

如果属性为true,则可以通过Value属性访问当前Nullable<T>对象的值。 HasValueIf the HasValue property is true, the value of the current Nullable<T> object can be accessed with the Value property. 否则,尝试访问其值会引发InvalidOperationException异常。Otherwise, attempting to access its value throws an InvalidOperationException exception.

适用于