This time, it is not as obvious to uses as the other new features of WPF 4.5.<\/p>\n

To be a MarkupExtension, a class have to inherit from MarkupExtesion and to implement the abstract ProvideValue <\/em>method. It is called by the framework which provide an IServiceProvider object as a parameter.<\/p>\n

This serviceProvider is a dependency resolver which you can use to retrieve a service named IProvideValueTarget<\/em>. This one will then be used to obtain the property targeted by the MarkupExtension(you can get the Targeted object too with it). <\/p>\n

This property is the Event’s accessor (the one which is called when you subscribe to it with the ‘+=’ syntax). Then, reflection has to be used to find the Type of the handler of the aimed event.<\/p>\n

Once this is done, a Delegate can be created and returned as a provided value by this MarkupExtension. In the example below, the delegate handler is a method named ‘MyMarkupExtensionInternalHandler’ defined on the MarkupExtension itself. <\/p>\n

[csharp]public override object ProvideValue(IServiceProvider serviceProvider)
\n{
\n IProvideValueTarget targetProvider = serviceProvider
\n .GetService(typeof(IProvideValueTarget)) as IProvideValueTarget;
\n if (targetProvider == null)
\n throw new InvalidOperationException(@"The CallAction extension
\n\t can’t retrieved the IProvideValueTarget service.");<\/p>\n

var targetEventAddMethod = targetProvider.TargetProperty as MethodInfo;<\/p>\n

\/\/Retrieve the handler of the event
\n ParameterInfo[] pars = targetEventAddMethod.GetParameters();
\n Type delegateType = pars[1].ParameterType;<\/p>\n

\/\/Retrieves the method info of the proxy handler
\n MethodInfo methodInfo = this.GetType()
\n .GetMethod("MyMarkupExtensionInternalHandler",
\n BindingFlags.NonPublic | BindingFlags.Instance);<\/p>\n

\/\/Create a delegate to the proxy handler on the markupExtension
\n Delegate returnedDelegate = Delegate
\n .CreateDelegate(delegateType, this, methodInfo);<\/p>\n

return returnedDelegate;
\n}<\/p>\n

void MyMarkupExtensionInternalHandler(object sender, EventArgs e)
\n{
\n\t\t\/\/here something can be performed.
\n}[\/csharp]<\/p>\n

A few good things to know when you create your own markup extensions:<\/p>\n