crossthread operations error

Question

      if (listBox1.InvokeRequired)
       {
           listBox = new StringBuilder(this.listBox1.Text);
       }

This is the code in c# which when executed produces an invalid cross thread operation error for listBox1 which is a listbox in my form. Could u guys please tell me why?? I am using the invokeRequired method too and am not changing the contents of the listbox either.

Answer 1

InvokeRequired simply checks to see if Invoke is required. You found it's required, yet didn't call Invoke!

Answer 2

InvokeRequired only tells you that an Invoke is necessary in order to validly access the element. It doesn't make the access legal. You must use the invoke method to push the update to the appropriate thread

Action update = () => listbox = new StringBuilder(this.listBox1.Text);
if (listBox1.InvokeRequired) {
  listBox1.Invoke(update);
} else {
  update();
}

Answer 3

Your code should run when InvokeRequired is false

delegate void SetListBoxDelegate(); 

void SetListBox()
{
    if(!InvokeRequired)
    {
        listBox = new StringBuilder(this.listBox1.Text);
    } 
    else 
        Invoke(new SetListBoxDelegate(SetListBox)); 
}

Edit: Check out Making Windows Forms thread safe