Open a form on the UI thread from a background thread

Question

I am trying to open a Form from a background thread (I think) because when I call formName.Show(); is freezes that form (Not the main form).

Goal:

When the user receives a new message, popup a newMessageFrm with the new message for reply.

Problem:

The new Form locks.

Here is the code I am using:

static void OnNewMessage(object sender, S22.Xmpp.Im.MessageEventArgs e)
        {
            if(CheckIfFormIsOpen(e.Jid.ToString(), e.Message.ToString()) == true){

            } else
            {
                newMessageFrm tempMsg = new newMessageFrm(e.Jid.ToString());

                tempMsg._msgText = e.Jid.ToString() + ": " + e.Message.ToString();
                tempMsg.frmId = e.Jid.ToString();
                tempMsg.Show(); //This locks up the application
            }


        }

I am using Visual Studio 2015, C#, and S22.Xmpp (As you can see from the code.)

When this event fires off the form does popup, but then locks.

Please let me know if you need any more information.

use Invoke or BeginInvoke of any control/form you have accessible. Look at this: http://stackoverflow.com/questions/6708765/how-to-invoke-when-i-have-no-control-available — Gusman, Jan 06 '16 at 19:02
PLease don't answer questions inside comments. After some time comments get deleted, also a user can not mark comments as answers — Stralos, Jan 06 '16 at 19:03
@Gusman: Seen that thread, it is unfortunately not what I need because the outcome is the same as above... — Mitchell Urgero, Jan 06 '16 at 19:10
Added the winforms tag as I'm sure you are talking about a windows forms application and not about wpf. — René Vogt, Jan 06 '16 at 19:11
@HamletHakobyan : How do I go about doing that from a background thread....? Which is my question. — Mitchell Urgero, Jan 06 '16 at 19:12
If it's WPF then it's even easier, just use the Dispatcher: Application.Current.Dispatcher.Invoke(new Action(() => { /* Your code here */ })); — Gusman, Jan 06 '16 at 19:12
And if it is not WPF? I have done research on this all morning and cannot find what I need.. I can launch the forms in a background thread from UI, but I need it the other way around... — Mitchell Urgero, Jan 06 '16 at 19:20
@Stralos It's perfectly valid to help the OP by answering in the comments. — LarsTech, Jan 06 '16 at 19:36

René Vogt · Accepted Answer · 2016-01-06T19:33:03.810

Unfortunately I don't know how to do this without any existing form. But I'm sure you have some kind of main form you can access. Or you can obtain a Form using

var invokingForm = Application.OpenForms[0];

So you can change the code of your method like this:

static void OnNewMessage(object sender, S22.Xmpp.Im.MessageEventArgs e)
{
    var invokingForm = Application.OpenForms[0]; // or whatever Form you can access
    if (invokingForm.InvokeRequired)
    {
        invokingForm.BeginInvoke(new EventHandler<S22.Xmpp.Im.MessageEventArgs>(OnNewMessage), sender, e);
        return; // important!!!
    }

    // the rest of your code goes here, now running
    // on the same ui thread as invokingForm
    if(CheckIfFormIsOpen(e.Jid.ToString(), e.Message.ToString()) == true)
    {
    }
    else
    {
        newMessageFrm tempMsg = new newMessageFrm(e.Jid.ToString());
        tempMsg._msgText = e.Jid.ToString() + ": " + e.Message.ToString();
        tempMsg.frmId = e.Jid.ToString();
        tempMsg.Show();
    }
}

Note that I assumend that S22.Xmpp.Im.MessageEventArgs is inherited from System.EventArgs.

score 3 · Answer 2 · answered Jan 06 '16 at 20:54

In WinForms there is still a requirement that the UI objects need to be on a thread with a message pump. This is usually the main application thread, also called the UI thread.

In Winforms checking to see if you are on the right thread to access a UI object is done with Control.InvokeRequired. If this returns true, it means you are not on the proper thread, and need to use an Invoke operation.

Since you know you are one a different thread, you don't need to check, you can just Invoke.

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    private void button1_Click(object sender, EventArgs e)
    {
        ThreadPool.QueueUserWorkItem(BadBackgroundLaunch);
        ThreadPool.QueueUserWorkItem(GoodBackgroundLaunch);
    }

    private void GoodBackgroundLaunch(object state)
    {
        this.Invoke((Action) (() =>
        {
            var form2 = new SecondForm();
            form2.Text = "Good One";
            form2.Show();
        }));
    }

    private void BadBackgroundLaunch(object state)
    {
        var form2 = new SecondForm();
        form2.Text = "Bad one";
        form2.Show();
    }
}

How do you know OP's method is declared inside a `Form` class? He only showed that it's a _static_ method. So you left out on which `Control` he should call `Invoke`. — René Vogt, Jan 06 '16 at 21:42
Any `Control` in the application will do. They should all be on the UI thread. Your approach `Application.OpenForms[0]` is fine. — bigtlb, Jan 06 '16 at 21:46
Yes, that is why I supposed that finding a control is part of the question. — René Vogt, Jan 06 '16 at 21:49
The question doesn't specify that the OP can't find an existing control. — bigtlb, Jan 06 '16 at 21:51

Open a form on the UI thread from a background thread

2 Answers2