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DynamicObject.TrySetMember Method

Provides the implementation for operations that set member values. Classes derived from the DynamicObject class can override this method to specify dynamic behavior for operations such as setting a value for a property.

Namespace:  System.Dynamic
Assembly:  System.Core (in System.Core.dll)

public virtual bool TrySetMember(
	SetMemberBinder binder,
	Object value
)

Parameters

binder
Type: System.Dynamic.SetMemberBinder

Provides information about the object that called the dynamic operation. The binder.Name property provides the name of the member to which the value is being assigned. For example, for the statement sampleObject.SampleProperty = "Test", 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.

value
Type: System.Object

The value to set to the member. For example, for sampleObject.SampleProperty = "Test", where sampleObject is an instance of the class derived from the DynamicObject class, the value is "Test".

Return Value

Type: 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 language-specific run-time exception is thrown.)

Classes derived from the DynamicObject class can override this method to specify how operations that set a value to a member 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 language-specific run-time exception is thrown.)

This method is called when you have statements like sampleObject.SampleProperty = "Test", where sampleObject is an instance of the class that is 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 TryGetMember 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 : 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

.NET Framework

Supported in: 4.5, 4

.NET Framework Client Profile

Supported in: 4

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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