C# callback from DLL

Question

I'm writing Application A and DLL B, both in C#.NET. How do I do the following:

1. A calls function in B
2. Want B to use delegate/callback to update status in UI of A

This is not about BackgroundWorker...that part works fine in A. What I can't see is how to let B know what function to call in A.

Answer 1

You have two options. The most common is to have an event in B and have your UI in A subscribe to that event. B then fires that event.

The second option is to pass in a delegate from A as a parameter to the method call in B. B can then Invoke that delegate.

Answer 2

Pass in the callback object in the call A make to B. Use an interface (or tightly bound libraries). Make sure the callback object is thread aware and thread safe.

Answer 3

To expand on Rob Prouse's answer, you need to declare a delegate and then pass a matching method into it.

In B:

public delegate void CallbackDelegate(string status);

public void DoWork(string param, CallbackDelegate callback)
{
    callback("status");
}

In A:

public void MyCallback(string status)
{
    // Update your UI.
}

And when you call the method:

B.DoWork("my params", MyCallback);

Answer 4

If you control B, then Rob Prouse or Brody's answers will work fine.

But what if you can't change B at all? In that case, you can always wrap a method in a delegate of your own making, as long it's signature matches that of the signature of the target method.

So, say you have a class instance named B with a public method named b() (from the B dll assembly, of course). Class A in the A application can call it asynchronously like this:

public class A
{
    delegate void BDelegate();

    public void BegineBMethod()
    {
        BDelegate b_method = new BDelegate(B.b);
        b_method.BeginInvoke(BCallback, null);
    }

    void BCallback(IAsyncResult ar)
    {
       // cleanup/get return value/check exceptions here
    }
}