Question

In a given form, the code is referencing a static element on the main form and updating it.

SomeRandomForm.cs

MainForm.Instance.UpdateSomeLabel("hello world");

Now it isn’t obvious if this code that is updating the main form’s static instance of a label (or whatever it may be) is on the UI thread or not.

Is it possible for me to wrap this method call (UpdateSomeLabel) somehow to ensure it is being updated on the UI thread only? I want to actually modify the method UpdateSomeLabel to ensure this updated only happends on a UI thread.

Just trying to clean-up this app to make it safe.

Answer 1

You should familiarize yourself with the Invoke(…) and BeginInvoke(…) methods and determine what semantics you need for this cross-thread update. Do you want to fire-and-forget, and not wait for the message pump to get back around to your text update? Use BeginInvoke(...). Do you want the thread to block until the UI update has occurred? Use Invoke(...).

Also, bear in mind that if the form may potentially be disposed, or hasn’t been shown yet, you need to check more than just the InvokeRequired flag on the form before calling Invoke(...) or BeginInvoke(...). If there is a potential for this case, your logic should look something like this:

public void DoSomeUpdate(string text)
{
    if(this.IsDisposed || (!this.IsHandleCreated))
    {
        // error condition; run away!
        return;
    }

    if(this.InvokeRequired)
    {
        // BeginInvoke or Invoke here, based on the semantics you need
        return;
    }

    this.UpdateSomeLabel(text);
}

Comment please if anything here is unclear.

Update

There has been some commentary regarding checking for whether the control’s handle has been created or if it has been disposed.

Regarding the former case (IsHandleCreated), that’s definitely something you can assume beforehand in many cases; however, not all cases. If you’re not dynamically creating controls, and your background thread stuff is being triggered via the UI thread, you’re probably safe. However…

If there’s any hint of a possibility that the background thread task could be in-flight long enough for you to click the ‘X’ on your form and close it (thereby disposing it), then you’ll get a nice exception if you don’t check IsDisposed before calling Invoke(...) or BeginInvoke(...). It’s similar to (in C) checking for malloc(...) returning something non-zero. It does happen. As tedious as it is, an application author is obligated to care about these potential error cases. This does add complication; however, it’s important, especially if you don’t want your app to crash. Failing to properly account for IsDisposed and IsHandleCreated when calling Invoke(...) or BeginInvoke(...) exposes you to the possibility of crashing your app. You can avoid that crash with a few extra lines of code.