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In my last post I talked about using funcs in C# to do async calls in WinForms to free up the main thread for the UI. In that post I demonstrated calling a method and then waiting until the value came back. Today I want to talk about calling a method and then continuing on and handling the results of the async call in a callback.

The difference is that in the previous example although the UI would not lock up the user couldn't really do anything while the other thread was working because it was waiting for it to finish. This time I want to allow the user to continue to do other stuff while waiting for the thread to finish.

Like before I have a service call I want to make that takes a long time to finish defined in a method called MyServiceCall. We need to define a callback method takes an IAsyncResult parameter.

public ServiceCallResult MyServiceCall(int param1)...

public int MyCallbackMethod(IAsyncResult ar)...

We start the same way by defining a delegate to the service call method using a Func. We need to pass an AsyncCallback object into the BeginInvoke method. This will tell it to call our callback method when MyServiceCall finishes. The second parameter to BeginInvoke is the Func delegate. This will give us access to it in our callback.

Func<int, ServiceCallResult> f = MyServiceCall;
AsyncCallback callback =
   new AsyncCallback(MyCallbackMethod);
IAsyncResult async = f.BeginInvoke(23, callback, f);

Now let's expand the callback method. The IAsyncResult parameter contains the Func delegate in its AsyncState property. We call EndInvoke on that Func to get the return value.

public int MyCallbackMethod(IAsyncResult ar)
    Func<int, ServiceCallResult> delegate =
        (Func<int, ServiceCallResult>)ar.AsyncState;
    ServiceCallResult result = delegate.EndInvoke(ar);

There you have it. Now you don't have to make the user wait for something that isn't critical to the loading of the page.
Posted on Wednesday, September 26, 2012 1:31 PM | Back to top

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