What's the difference between Invoke() and BeginInvoke()

Question

Just wondering what the difference between BeginInvoke() and Invoke() are?

Mainly what each one would be used for.

EDIT: What is the difference between creating a threading object and calling invoke on that and just calling BeginInvoke() on a delegate? or are they the same thing?

Thanks

Answer 1

Do you mean Delegate.Invoke/BeginInvoke or Control.Invoke/BeginInvoke?

• Delegate.Invoke: Executes synchronously, on the same thread.
• Delegate.BeginInvoke: Executes asynchronously, on a threadpool thread.
• Control.Invoke: Executes on the UI thread, but calling thread waits for completion before continuing.
• Control.BeginInvoke: Executes on the UI thread, and calling thread doesn't wait for completion.

Tim's answer mentions when you might want to use BeginInvoke - although it was mostly geared towards Delegate.BeginInvoke, I suspect.

For Windows Forms apps, I would suggest that you should usually use BeginInvoke. That way you don't need to worry about deadlock, for example - but you need to understand that the UI may not have been updated by the time you next look at it! In particular, you shouldn't modify data which the UI thread might be about to use for display purposes. For example, if you have a Person with FirstName and LastName properties, and you did:

person.FirstName = "Kevin"; // person is a shared reference
person.LastName = "Spacey";
control.BeginInvoke(UpdateName);
person.FirstName = "Keyser";
person.LastName = "Soze";

then the UI may well end up displaying "Keyser Spacey". (There's an outside chance it could display "Kevin Soze" but only through the weirdness of the memory model.)

Unless you have this sort of issue, however, Control.BeginInvoke is easier to get right, and will avoid your background thread from having to wait for no good reason. Note that the Windows Forms team has guaranteed that you can use Control.BeginInvoke in a "fire and forget" manner - i.e. without ever calling EndInvoke. This is not true of async calls in general: normally every BeginXXX should have a corresponding EndXXX call, usually in the callback.

Answer 2

Delegate.BeginInvoke() asynchronously queues the call of a delegate and returns control immediately. When using Delegate.BeginInvoke(), you should call Delegate.EndInvoke() in the callback method to get the results.

Delegate.Invoke() synchronously calls the delegate in the same thread.

MSDN Article

Answer 3

Building on Jon Skeet's reply, there are times when you want to invoke a delegate and wait for its execution to complete before the current thread continues. In those cases the Invoke call is what you want.

In multi-threading applications, you may not want a thread to wait on a delegate to finish execution, especially if that delegate performs I/O (which could make the delegate and your thread block).

In those cases the BeginInvoke would be useful. By calling it, you're telling the delegate to start but then your thread is free to do other things in parallel with the delegate.

Using BeginInvoke increases the complexity of your code but there are times when the improved performance is worth the complexity.

Answer 4

What is thoroughly unclear and has been driving me nuts is why you need to pass back the Iasyncresult when getting the result

See here - Why does IAsyncResult have to get passed into endinvoke???

    IAsyncResult ar = del.BeginInvoke(out start, out stop,null, null);
    ar.AsyncWaitHandle.WaitOne();

    int elapse = del.EndInvoke(out start, out stop, ar);

Answer 5

I'm confused at Jon's reply:

Control.Invoke: Executes on the UI thread, but calling thread waits for completion before continuing. Control.BeginInvoke: Executes on the UI thread, and calling thread doesn't wait for completion

I don't think Control.Invoke/BeginInvoke executes on the UI thread, they should execute on the thread that owns the Control's handle.( which is not always the UI thread ). See MSDN