Question

In an asynchronous OnMsgRecieved call, if I assign a value directly to a control it is not working.
Then i came to know that it was due to thread unsafe and i got following code to resolve the issue.
Now it is working. But i am not sure what it does practically. Can any one make me to understand it fully?
The code is:-

        public void listener_OnMsgRecieved(string aResponse)
    {
        ShowResponseMessage(aResponse);
    }

    public void ShowResponseMessage(string aResponse)
    {
        // InvokeRequired required compares the thread ID of the
        // calling thread to the thread ID of the creating thread.
        // If these threads are different, it returns true.
        if (this.listBox.InvokeRequired)
        {
            SetTextCallback d = new SetTextCallback(ShowResponseMessage);
            this.Invoke(d, new object[] { aResponse });
        }
        else
        {
            this.listBox.Items.Add(aResponse);
            label.Text = "Response received from Server :";
        }
    }

Answer 1

When ShowResponseMessage is called on a different thread from that of the UI, the InvokeRequired will return true, then you are using Control.Invoke to send a message to the Windows message queue.

The UI message pump which runs in the UI thread will pull the message and deliver it to the target control, the target control then sees that this is a message requesting that a delegate be invoked and the delegate is invoke by the control, this is now running on the UI thread and therefore the cross threading issue has been resolved.

The trick is that the delegate is not called directly on the non-UI calling thread. Using Windows messages the instruction to execute the delegate is passed to the UI thread which then executes the delegate in response to the message. ‘Control.Invoke’ uses the Windows [SendMessage][1], Control.BeginInvoke uses the [PostMessage][2] Win32 API to facilitate the message passing.