Question

I’m pretty new at C# so forgive me if this is a bit of a silly question.
Anyway I’m writing a little chat application that so far seems to be going well, I’ve run into a problem that I seem to have fixed but I’m hoping to work out why I need to do it this way.

When I receive a message I want to update a text box so I use:

txtConnectedID.Text = "test";

But I receive this error:

System.InvalidOperationException was unhandled by user code
  Message=Cross-thread operation not valid: Control 'txtConnectedID' accessed from a thread other than the thread it was created on.

Now, I think this has something to do with stopping the method running twice and not updating properly? I’m not 100% on this. So now I have a delegate for a method that accepts a string and I call:

private delegate void stringDelegate(string s);

BeginInvoke(new stringDelegate(writeToIPBox), new object[] { e.ConnectedIP.ToString() });

private void writeToIPBox(string newIP)
        {
            txtConnectedID.Text = newIP;
        }

I’m not sure why I’m doing this, how it’s any different. I’m not really happy to just do it this way without knowing why.

Thanks in advance

Answer 1

You should only attempt to update controls from the thread on which they were created. There are good reasons for this as it is easy to hit a race condition. These controls are not thread safe and this is the runtime helping you out a bit.

Instead, as you have figured out, you need to update it on the UI thread, which is what BeginInvoke is doing; calling the delegate asynchronously on the UI thread.