Question

I have a windows form with a text box txtOutput. I have some content in it. I have written a property to get and set the txtOutput.Text both from within the same thread and across threads like this:

public string OutputString
{
    get
    {
        string text = string.Empty;
        if (txtOutput.InvokeRequired)
        {
            txtOutput.BeginInvoke(new MethodInvoker(delegate
                {
                    text = txtOutput.Text;
                }));
        }
        else
        {
            text = txtOutput.Text;
        }
        return text;
    }
    set
    {
        if (txtOutput.InvokeRequired)
        {
            txtOutput.BeginInvoke(new MethodInvoker(delegate
                {
                    txtOutput.Text = value;
                }));
        }
        else
        {
            txtOutput.Text = value;
        }
    }
}

If I set/get the property from the same thread, the behavior is as expected when calling the below function like PrintMessage().

public void PrintMessage()
{
    MessageBox.Show(OutputString);
}

But when I call like this new Thread(PrintMessage).Start(). The get does not retrieve the value in the text box (i.e., the MessageBox shows empty string). When I do the same by keeping a breakpoint on the line:

txtOutput.BeginInvoke(new MethodInvoker(delegate
{
    text = txtOutput.Text;
}));

while debug, the value is retrieved (i.e., the MessageBox shows the txtOutput content)

Should I sleep somewhere? Where am I making the mistake?

Answer 1

The problem is you’re calling MessageBox.Show() with a reference to the text variable before the UI thread can handle the request you placed with the dispatcher. I would avoid using Thread.Sleep() as you could end up with some nasty side effects. Ideally you should re-factor your code to get rid of the property, which is synchronous, in favor of a more asynchronous solution. Something similar to the code below should give you the result you’re looking for:

public void PrintMessage(Action<string> displayAction)
{
    string text = string.Empty;
    if (txtOutput.InvokeRequired)
    {
        txtOutput.BeginInvoke(new MethodInvoker(delegate
        {
            displayAction.Invoke(txtOutput.Text);
        }));
    }
    else
    {
        displayAction.Invoke(txtOutput.Text);
    }
}

And Invoke it:

// Get the text asynchronously
PrintMessage(s => MessageBox.Show(s));