Invoke from different thread

Question

I can't believe it,this works in my other application,but in this one,which has similiar structure - it doesn't!

    public string ListAdd
    {
        set
        {
            if (listView1.InvokeRequired)
            {
                this.Invoke((MethodInvoker)delegate
                {
                    listView1.Items.Add(value);
                });
            }
            else
            {
                listView1.Items.Add(value);
            }
        }
    }

The way I call it:

        var formz = Form.ActiveForm as Form1;
        if (formz != null)
            formz.ListAdd = "asdasdasdasd";

If I call it from the function where I receive packet(I want to put it on the listView) over TCP - Nothing happens,but If I call it in that same class where I initialize WinSock - it works.

Where could the problem be?

EDIT: I just debugged the application ,formz is null at the place I call it(receive function). Why is it null there,but not at any other place?

Answer 1

If ActiveForm is returning null then you might not have an active form or it is not of type Form1. You are using "as Form1", so if you have a Form2 which is active then formz will be set to null.

Can you pass formz into the function instead of calling ActiveForm?

Answer 2

I do not think, it will solve your problem, but have you thought about using the following pattern to do the invoke? I consider it much more readable. Create this extension method.

public static class ControlExtension
{
   public static void ThreadSafeInvoke(this Control control, MethodInvoker method)
   {
      if (control != null)
      {
         if (control.InvokeRequired)
         {
            control.Invoke(method);
         }
         else
         {
            method.Invoke();
         }
      }
   }
}

And then you can perform thread safe method calls like this.

Form form = new Form();

form.ThreadSafeInvoke(() => form.Text = "ThreadSafeInvoke");

Or multiple calls at once.

form.ThreadSafeInvoke(() =>                 
{
   form.Text = "ThreadSafeInvoke";
   form.Visible = true;
   form.WindowState = FormWindowState.Maximized;
});

UPDATE

So the problem is clearly Form.ActiveForm returning null.

1. There is no active form at the moment of the call.
2. The thread has no permission to get the active form - MSDN states that UIPermission is required.