DynamicObject.TryGetMember(GetMemberBinder, Object) 方法

定义

命名空间:

System.Dynamic

程序集:

System.Linq.Expressions.dll

Source:

DynamicObject.cs

为获取成员值的操作提供实现。 从 DynamicObject 类派生的类可以重写此方法，以便为诸如获取属性值这样的操作指定动态行为。

public:
 virtual bool TryGetMember(System::Dynamic::GetMemberBinder ^ binder, [Runtime::InteropServices::Out] System::Object ^ % result);

public virtual bool TryGetMember (System.Dynamic.GetMemberBinder binder, out object? result);

abstract member TryGetMember : System.Dynamic.GetMemberBinder * obj -> bool
override this.TryGetMember : System.Dynamic.GetMemberBinder * obj -> bool

Public Overridable Function TryGetMember (binder As GetMemberBinder, ByRef result As Object) As Boolean

参数

binder

GetMemberBinder

提供有关调用了动态操作的对象的信息。 属性 binder.Name 提供对其执行动态操作的成员的名称。 例如，对于 Console.WriteLine(sampleObject.SampleProperty) 语句，其中 sampleObject 是派生自 DynamicObject 类的类的实例， binder.Name 返回“SampleProperty”。 属性 binder.IgnoreCase 指定成员名称是否区分大小写。

result

Object

获取操作的结果。 例如，如果为某个属性调用该方法，则可以为 result 指派该属性值。

返回

Boolean

如果操作成功，则为 true；否则为 false。 如果此方法返回 false，则该语言的运行时联编程序将决定行为。 （大多数情况下，将引发运行时异常。）

示例

假设你想要提供替代语法来访问字典中的值，这样就可以编写 (，而不是sampleDictionary["Text"] = "Sample text"在 Visual Basic) 中编写 sampleDictionary.Text = "Sample text"。sampleDictionary("Text") = "Sample text" 此外，此语法必须不区分大小写，以便等效 sampleDictionary.Text 于 sampleDictionary.text。

下面的代码示例演示 DynamicDictionary 派生自 类的 DynamicObject 类。 类DynamicDictionary包含 Visual Basic) 中 (Dictionary(Of String, Object) 类型的 对象Dictionary<string, object>，用于存储键值对，并重写 TrySetMember 和 TryGetMember 方法以支持新语法。 它还提供一个 Count 属性，该属性显示字典包含的动态属性数。

// The class derived from DynamicObject.
public class DynamicDictionary : DynamicObject
{
    // The inner dictionary.
    Dictionary<string, object> dictionary
        = new Dictionary<string, object>();

    // This property returns the number of elements
    // in the inner dictionary.
    public int Count
    {
        get
        {
            return dictionary.Count;
        }
    }

    // If you try to get a value of a property
    // not defined in the class, this method is called.
    public override bool TryGetMember(
        GetMemberBinder binder, out object result)
    {
        // Converting the property name to lowercase
        // so that property names become case-insensitive.
        string name = binder.Name.ToLower();

        // If the property name is found in a dictionary,
        // set the result parameter to the property value and return true.
        // Otherwise, return false.
        return dictionary.TryGetValue(name, out result);
    }

    // If you try to set a value of a property that is
    // not defined in the class, this method is called.
    public override bool TrySetMember(
        SetMemberBinder binder, object value)
    {
        // Converting the property name to lowercase
        // so that property names become case-insensitive.
        dictionary[binder.Name.ToLower()] = value;

        // You can always add a value to a dictionary,
        // so this method always returns true.
        return true;
    }
}

class Program
{
    static void Main(string[] args)
    {
        // Creating a dynamic dictionary.
        dynamic person = new DynamicDictionary();

        // Adding new dynamic properties.
        // The TrySetMember method is called.
        person.FirstName = "Ellen";
        person.LastName = "Adams";

        // Getting values of the dynamic properties.
        // The TryGetMember method is called.
        // Note that property names are case-insensitive.
        Console.WriteLine(person.firstname + " " + person.lastname);

        // Getting the value of the Count property.
        // The TryGetMember is not called,
        // because the property is defined in the class.
        Console.WriteLine(
            "Number of dynamic properties:" + person.Count);

        // The following statement throws an exception at run time.
        // There is no "address" property,
        // so the TryGetMember method returns false and this causes a
        // RuntimeBinderException.
        // Console.WriteLine(person.address);
    }
}

// This example has the following output:
// Ellen Adams
// Number of dynamic properties: 2

' The class derived from DynamicObject.
Public Class DynamicDictionary
    Inherits DynamicObject

    ' The inner dictionary.
    Dim dictionary As New Dictionary(Of String, Object)

    ' This property returns the number of elements
    ' in the inner dictionary.
    ReadOnly Property Count As Integer
        Get
            Return dictionary.Count
        End Get
    End Property


    ' If you try to get a value of a property that is
    ' not defined in the class, this method is called.

    Public Overrides Function TryGetMember(
        ByVal binder As System.Dynamic.GetMemberBinder,
        ByRef result As Object) As Boolean

        ' Converting the property name to lowercase
        ' so that property names become case-insensitive.
        Dim name As String = binder.Name.ToLower()

        ' If the property name is found in a dictionary,
        ' set the result parameter to the property value and return true.
        ' Otherwise, return false.
        Return dictionary.TryGetValue(name, result)
    End Function

    Public Overrides Function TrySetMember(
        ByVal binder As System.Dynamic.SetMemberBinder,
        ByVal value As Object) As Boolean

        ' Converting the property name to lowercase
        ' so that property names become case-insensitive.
        dictionary(binder.Name.ToLower()) = value

        ' You can always add a value to a dictionary,
        ' so this method always returns true.
        Return True
    End Function
End Class

Sub Main()
    ' Creating a dynamic dictionary.
    Dim person As Object = New DynamicDictionary()

    ' Adding new dynamic properties.
    ' The TrySetMember method is called.
    person.FirstName = "Ellen"
    person.LastName = "Adams"

    ' Getting values of the dynamic properties.
    ' The TryGetMember method is called.
    ' Note that property names are now case-insensitive,
    ' although they are case-sensitive in C#.
    Console.WriteLine(person.firstname & " " & person.lastname)

    ' Getting the value of the Count property.
    ' The TryGetMember is not called, 
    ' because the property is defined in the class.
    Console.WriteLine("Number of dynamic properties:" & person.Count)

    ' The following statement throws an exception at run time.
    ' There is no "address" property,
    ' so the TryGetMember method returns false and this causes
    ' a MissingMemberException.
    ' Console.WriteLine(person.address)
End Sub
' This examples has the following output:
' Ellen Adams
' Number of dynamic properties: 2

注解

派生自 类的 DynamicObject 类可以重写此方法，以指定应如何为动态对象执行获取成员值的操作。 当方法未重写时，语言的运行时绑定器将确定行为。 （大多数情况下，将引发运行时异常。）

如果具有 等 Console.WriteLine(sampleObject.SampleProperty)语句，则调用此方法，其中 sampleObject 是派生自 类的类的 DynamicObject 实例。

还可以将自己的成员添加到派生自 类的类。DynamicObject 如果类定义属性并重写 TrySetMember 方法，则动态语言运行时 (DLR) 首先使用语言绑定器来查找类中属性的静态定义。 如果没有此类属性，DLR 将调用 TrySetMember 方法。