DynamicObject.TryGetMember Method (GetMemberBinder, Object)
Provides the implementation for operations that get member values. Classes derived from the DynamicObject class can override this method to specify dynamic behavior for operations such as getting a value for a property.
Assembly: System.Core (in System.Core.dll)
Public Overridable Function TryGetMember ( binder As GetMemberBinder, <OutAttribute> ByRef result As Object ) As Boolean
Provides information about the object that called the dynamic operation. The binder.Name property provides the name of the member on which the dynamic operation is performed. For example, for the Console.WriteLine(sampleObject.SampleProperty) statement, where sampleObject is an instance of the class derived from the DynamicObject class, binder.Name returns "SampleProperty". The binder.IgnoreCase property specifies whether the member name is case-sensitive.
The result of the get operation. For example, if the method is called for a property, you can assign the property value to result.
Return ValueType: System.Boolean
true if the operation is successful; otherwise, false. If this method returns false, the run-time binder of the language determines the behavior. (In most cases, a run-time exception is thrown.)
Classes derived from the DynamicObject class can override this method to specify how operations that get member values should be performed for a dynamic object. When the method is not overridden, the run-time binder of the language determines the behavior. (In most cases, a run-time exception is thrown.)
This method is called when you have statements like Console.WriteLine(sampleObject.SampleProperty), where sampleObject is an instance of the class derived from the DynamicObject class.
You can also add your own members to classes derived from the DynamicObject class. If your class defines properties and also overrides the TrySetMember method, the dynamic language runtime (DLR) first uses the language binder to look for a static definition of a property in the class. If there is no such property, the DLR calls the TrySetMember method.
Assume that you want to provide alternative syntax for accessing values in a dictionary, so that instead of writing sampleDictionary["Text"] = "Sample text" (sampleDictionary("Text") = "Sample text" in Visual Basic), you can write sampleDictionary.Text = "Sample text". Also, this syntax must be case-insensitive, so that sampleDictionary.Text is equivalent to sampleDictionary.text.
The following code example demonstrates the DynamicDictionary class, which is derived from the DynamicObject class. The DynamicDictionary class contains an object of the Dictionary<string, object> type (Dictionary(Of String, Object) in Visual Basic) to store the key-value pairs, and overrides the TrySetMember and methods to support the new syntax. It also provides a Count property, which shows how many dynamic properties the dictionary contains.
' 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
Available since 8
Available since 4.0
Portable Class Library
Supported in: portable .NET platforms
Available since 4.0
Windows Phone Silverlight
Available since 8.0
Available since 8.1