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AsyncCallback is a means of associating a delegate with the asynchronous delegate. When the asynchronous delegate completes its execution, the method associated with AsyncCallback is invoked. That method can then call EndInvoke and retrieve
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output values or the return code. In the following code example, we start execution of MyDelegate in the TestCallback method. When we call BeginInvoke we pass in CompleteCb as the second parameter. The method associated with CompleteCb, Complete, is executed as soon as TestMethod completes its execution.
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Private Sub TestCallback() Dim state As Object = "" Dim TheAsyncResult As IAsyncResult Dim MyDelegate As TestDelegate MyDelegate = New TestDelegate(AddressOf TestMethod) Dim CompleteCb As AsyncCallback CompleteCb = New AsyncCallback(AddressOf Complete) TheAsyncResult = MyDelegate.BeginInvoke(state, CompleteCb, Nothing) Trace.WriteLine("Exiting TestCallback") End Sub Private Sub Complete (ByVal TheAsyncResult As IAsyncResult) Dim TheResults As AsyncResult = CType(TheAsyncResult, AsyncResult) Dim ReturnValue As String Dim state As Object = "" Dim MyDelegate As TestDelegate MyDelegate = CType(TheResults.AsyncDelegate, TestDelegate) ReturnValue = MyDelegate.EndInvoke(state, TheAsyncResult) Trace.Write(ReturnValue) Trace.WriteLine(CType(state, String)) End Sub
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Complete is invoked as soon as the asynchronous execution is complete. It calls EndInvoke and retrieves both the output parameters and the return value of TestMethod. The last parameter in the BeginInvoke method is an object that is passed through to the IAsyncResult object. It is available from the AsyncResult object by accessing the AsyncState property.
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Suppose you know that at some point you need to execute one of five delegates. One way to do that would be to create a large case statement and create each of the delegates. Another alternative is to use the CreateDelegate method of the Delegate class. CreateDelegate allows for late binding. It allows a developer to determine at runtime what method is associated with a particular delegate, along with an optional target. The target is the same as the target from the previous sections in this chapter; it is an instance of a class that the method belongs to. In the following example the target is the current class, referenced by the this keyword:
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. . . void TestMethod4() { StackTrace MyTrace=new StackTrace (); Trace.WriteLine(MyTrace.GetFrame(0).GetMethod().Name); } delegate void TestDelegate(); private void buttonTest_Click(object sender, System.EventArgs e) { Delegate MyDelegate; string[] Methods = { "TestMethod0", "TestMethod1" , "TestMethod2", "TestMethod3","TestMethod4"}; Random Rnd=new Random(Environment.TickCount); string MethodToUse = Methods[Rnd.Next(Methods.Length)]; MyDelegate = Delegate.CreateDelegate(typeof(TestDelegate),this,MethodToUse ); MyDelegate.DynamicInvoke(null); } . . .
Once the delegate has been created we need some means of invoking it. The DynamicInvoke method allows for invocation of delegates that are created using the CreateDelegate method. It accepts an array of objects as its only parameter. These objects are the parameters, if any, that the method associated with the delegate expects.
CreateDelegate CreateDelegate is a static method of the delegate class. It creates a delegate
of a specified type and associates it with a target object and a method to invoke.
The late binding referred to in this section refers to binding a method, and option object, to a delegate. It should not be confused with other forms of late binding.
DynamicInvoke DynamicInvoke is a method of the delegate class. It allows delegates created
using CreateDelegate to be invoked. It accepts a single parameter, which is an array of objects that should correspond to the arguments of the method associated with the delegate.
There are many situations where late binding is a good idea. There are things that must be accounted for when doing late binding. One situation that can arise is that the target method referenced does not exist. In that case the following exception is raised:
An unhandled exception of type 'System.ArgumentException' occurred in mscorlib.dll Additional information: Error binding to target method.
The alternative to using CreateDelegate and DynamicInvoke is to use a large case statement:
private void UseCaseStatement() { TestDelegate MyDelegate=null; string[] Methods ={"TestMethod0","TestMethod1"};
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Random Rnd=new Random(Environment.TickCount); string MethodToUse = Methods[Rnd.Next(Methods.Length)]; switch(MethodToUse) { case "TestMethod0": MyDelegate = new TestDelegate(TestMethod0); break; case "TestMethod1": MyDelegate = new TestDelegate(TestMethod1); break; } if (MyDelegate != null) MyDelegate(); }
One of the biggest advantages of using this approach is that if references to nonexistent methods exist they will be caught at compile time rather than runtime. As with all things there are tradeoffs to both approaches and the situation will dictate which is the better approach.
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