Question

This code is executed by many way. When it’s executed by the form button it works (the button start a thread and in the loop it call this method = it works). BUT it doesn’t work when I have a call to that method from my BackgroundWorker in the form.

With the following code:

private void resizeThreadSafe(int width, int height) {     if (this.form.InvokeRequired)     {         this.form.Invoke(new DelegateSize(resizeThreadSafe),             new object[] { width, height });     }     this.form.Size = new Size(width, height); // problem occurs on this line     this.form.Location = new Point(0, 0); // dummy coordinate } 

Then on the line containing this.form.Size = ... I get the following exception:

InvalidOperationException was unhandled Cross-thread operation not valid: Control 'Form1' accessed from a thread other than the thread it was created on. 

Why?

Answer 1

You need to return at the end of the if block – otherwise you’ll resize it in the right thread, and then do it in the wrong thread as well.

In other words (if you’d cut and paste the code instead of a picture, this would have been easier…)

private void resizeThreadSafe(int width, int height) {     if (this.form.InvokeRequired)     {         this.form.Invoke(new DelegateSize(resizeThreadSafe,             new object[] { width, height });         return;     }     this.form.Size = new Size(width, height);     this.form.Location = new Point(0, SystemInformation.MonitorSize // whatever comes next } 

Alternatively just put the second half of the method in an ‘else’ block.