What is wrong with my InvokeRequied #2?

Question

The first topic is What wrong with my InvokeRequied

I followed the answer that he recommended it to me but I found a new problem.

The result of below picture is cross thread exception.

What is wrong with my code ?

How to solve this problem ?

Answer 1

According to MSDN InvokeRequired can return false even in cases where InvokeRequired should be true - namely in the case that you access InvokeRequired before the Handle of that control/form (or a parent of it) has been created.

Basically your check is incomplete which leads to the result you see.

You need to check IsHandleCreated - if that is false then you would need to use Invoke/BeginInvoke regardless of what InvokeRequired returns.

[UPDATE] BUT: This usually won't work robustly since Invoke/BeginInvoke check which thread created Handle to do their magic... [/UPDATE]

Only if IsHandleCreated is true you act based on what InvokeRequired returns - something along the lines of:

if (control.IsHandleCreated)
{
    if (control.InvokeRequired)
    {
        control.BeginInvoke(action);
    }
    else
    {
        action.Invoke();
    }
}
else 
{ 
    // in this case InvokeRequired might lie - you need to make sure that this never happens! 
    throw new Exception ( "Somehow Handle has not yet been created on the UI thread!" );
}

[UPDATE]

Thus the following is important to avoid this problem

Always make sure that the Handle is already created BEFORE the first access on a thread other than the UI thread.

According to MSDN you just need to reference control.Handle in the UI thread to force it being created - in your code this must happen BEFORE the very first time you access that control/form from any thread that is not the UI thread.

[/UPDATE]