How does C# evaluate Logic block??

Question

I have the following line of code:

    public bool dcpl_radar()
    {
        if (radar == null)
            return false;
        else
        {
            if (radar != null)
            {
                if (radar.InvokeRequired)
                    radar.BeginInvoke(new MethodInvoker(delegate() 
                                                { 
                                                    radar.Visible = false; 
                                                }));
                else
                    this.radar.Visible = false;

                radar = null;
            }
            return true;
        }//end of else statement
    }

but VStudio keeps throwing an error on the invoke line. I've checked the Debugger and if (radar == null) is true, yet VStudio is trying to evaluate a part of the code it shouldn't be in. Can someone explain why it's doing this please?

Answer 1

Wait a minute... I think we have a race condition. Lets say you BeginInvoke, almost immediately you set radar = null. There really is no telling when your anonymous delegate will be executed.

I would imagine this should solve your issue.

public bool dcpl_radar()
{
    if (radar != null)
    {
        if (radar.InvokeRequired)
        {
            radar.BeginInvoke(new MethodInvoker(HideRadar));
        }
        else
        {
            HideRadar();
        }

        return true;
    }

    return false;
}

private void HideRadar()
{
    this.radar.Visible = false;
    this.radar = null;
}

Answer 2

What is happening:

The anonymous delegate is being called after you set the radar to null.

How to fix it

public bool dcpl_radar()
{
    if (radar == null)
        return false;
    else
    {
        if (radar != null)
        {
            if (radar.InvokeRequired)
                radar.BeginInvoke(new MethodInvoker(delegate() 
                                            { 
                                                radar.Visible = false; 
                                                radar = null;
                                            }));
            else {
                this.radar.Visible = false;
                radar = null;
            }


        }
        return true;
    }//end of else statement
}

(Note where I've moved your 'null' assignments).

Though I am a bit worried about the point of setting a variable to null, it's generally a sign of a bad design.