Cross-thread operation not valid

Question

I keep getting the following error when debugging.

Cross-thread operation not valid: Control 'richTextBoxReceivedMsg' accessed from a thread other than the thread it was created on.

Here's the code that it points to:

public void OnDataReceived(IAsyncResult asyn)
{
    try
{
    SocketPacket socketData = (SocketPacket)asyn.AsyncState;

    int iRx  = 0;

        // Complete the BeginReceive() asynchronous call by EndReceive() method
        // which will return the number of characters written to the stream by the client
        iRx = socketData.m_currentSocket.EndReceive (asyn);

        char[] chars = new char[iRx +  1];
        System.Text.Decoder d = System.Text.Encoding.UTF8.GetDecoder();
        int charLen = d.GetChars(socketData.dataBuffer, 0, iRx, chars, 0);
        System.String szData = new System.String(chars);
        richTextBoxReceivedMsg.AppendText(szData);

        // Continue the waiting for data on the Socket
        WaitForData( socketData.m_currentSocket);
    }
    catch (ObjectDisposedException)
    {
        System.Diagnostics.Debugger.Log(0,"1","\nOnDataReceived: Socket has been closed\n");
    }
    catch (SocketException se)
    {
        MessageBox.Show(se.Message);
    }
}

Can somebody please help me fix this?

Solution

You need to replace this:

richTextBoxReceivedMsg.AppendText(szData);

with something like

Invoke(new Action(() => richTextBoxReceivedMsg.AppendText(szData)));

The reason is that Windows Forms is not really designed to work across different threads. Invoke method will run the delegate you pass to it in the UI thread. If you want to manipulate UI elements via other threads, you'll have to run the actual manipulation on the UI thread. InvokeRequired property will tell you when you need to use Invoke rather than calling the method directly.

Comments

[Admin]

Thank you, Mehrdad. :) - ... It displays a message saying "Cannot convvert anonymous method to type 'System.Delegate' because it is not a delegate."

[mmx]

@j-t-s: I always get this wrong and forget the argument to invoke is System.Delegate. This should fix it.