Control accessed from a thread other than the thread it was created on in C#

Question

So I have a button that lately will be capable of open a door through SerialPort. To do that I will send something to it and wait for a response from it which is already configured and developed.

But I am getting some trouble to set a Thread.Sleep().

This is what I got inside of the button:

private void cmdOpenDoor_Click(object sender, EventArgs e)
{
    Thread.Sleep(5000);
    Task.Factory.StartNew(this.Demo);
}

And the Demo method is:

private void Demo()
{
    string questionMark= "?";
    string incoming = comport.ReadExisting();
    string carriageReturn = "\r";

    comport.Write(questionMark+ "(" + carriageReturn);

    Cursor.Current = Cursors.WaitCursor;

    if (txtTest.Text == "@q")
    {
        MessageBox.Show("Door opened!", MessageBoxButtons.OK, MessageBoxIcon.Information);
        pbGreen.Show();
        pbRed.Hide();
     }
     else
     {
        MessageBox.Show("The door is already opened!", MessageBoxButtons.OK, MessageBoxIcon.Error);
     }
}

When I will open the door it shows the "Door opened" message and when it tries to execute the pbGreen.Show() the applications stops and an error comes:

Cross-thread operation not valid: Control accessed from a thread other than the thread it was created on

Do you have any idea how can I solve it? I cannot Invoke a PictureBox

Search for exact error message, e.g. [here](http://stackoverflow.com/q/142003/1997232) is duplicate for winforms and [here](http://stackoverflow.com/q/11923865/1997232) is for wpf. — Sinatr, Nov 14 '16 at 14:52
The problem is irrelevant to control type and yes, you can *invoke a `PictureBox`*. I would start with reading corresponding linked questions answers, if it's still not clear what you need to do, then: 1) add corresponding tag to the question (winforms? wpf? ...) 2) explain why you *"cannot Invoke"*. The message you receive should gone as soon as you dispatch/invoke GUI related stuff into GUI thread. — Sinatr, Nov 14 '16 at 15:01

Mark Phillips · Answer 1 · 2016-11-14T15:07:37.623

As the control you're trying to access (in this instance pbGreen and most likely pbRed as well) were created on the Main Thread, and you're trying to update them on a differnet thread, you get this error. To fix the issue, you need to make your calls thread-safe by invoking.

private void Demo()
{
    if (InvokeRequired) {
        Invoke(new MethodInvoker(() => { Demo(); }));
        return;
    }

    //Do stuff
}

However making your own method could be even better:

public static void InvokeIfRequired(this ISynchronizeInvoke formControl, MethodInvoker action)
{
    if (formControl.InvokeRequired) {
        formControl.Invoke(action, new object[0]);
    } else {
        action();
    }
}

Then:

pbGreen.InvokeIfRequired(() => { pbGreen.Image = myImage; });

Control accessed from a thread other than the thread it was created on in C#

1 Answers1