crossthread operations error

Question

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

This is the code in c# which when exec

Answer 1

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

Answer 2

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

Answer 3

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();
}