Sending an event from a thread to a UI

Question

Apologies as I know cross-thread operations have been addressed elsewhere, but all I see are fragments and can't quite figure out where to apply the solutions correctly. So I have distilled this problem to it's most basic essence in the hope that me and anybody else who comes across this can see a complete solution without a lot of support code to wade through or fragments with limited context. I have looked at various posts such as here, here and here but can't quite place the solutions in context.

I have created a Windows Forms project with a single button and textbox. In the form.cs file here is the complete code to demonstrate the problem:

using System;
using System.Windows.Forms;
using System.Threading;

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

    private void Button1_Click(object sender, EventArgs e)
    {
        new Thread(() =>
        {
            Thread.CurrentThread.IsBackground = true;
            TestClass();
        }).Start();
    }
    private void TestClass()
    {
        TestEvent tv = new TestEvent();
        tv.OnClsConnect += new TestEvent.clsConnect(OnConnected);
        tv.DoSomethingThread();
    }
    public void OnConnected(string str)
    {
        textBox1.Text = str;
    }
}
public class TestEvent
{
    public delegate void clsConnect(string str);
    public event clsConnect OnClsConnect;

    public void DoSomethingThread()
    {
        if (OnClsConnect != null)
        {
            OnClsConnect("Thread run");
        }
    }

}
}

Click the button and you will get the "Cross thread operation not valid" error. How does one properly fix this code? TIA

You have to tell that you want to execute code on UI thread. In order to do that you invoke method `Dispatcher.CurrentDispatcher.Invoke` which executes a job on UI thread. All of this is because UI elements can only be updated from UI thread. This is by design and you have to stick to it. — FCin, Sep 15 '19 at 18:11
The marked duplicate, which comprehensively addresses cross-thread scenarios in Winforms, has as its accepted and highest-voted answer, a code example that is no less direct and clear than the one you posted below. — Peter Duniho, Sep 15 '19 at 23:23
I looked at the example you posted. It is a code snippet, precisely what I was trying to avoid here. In addition mine has a separate class as opposed to just another thread. I respectfully disagree. — ds_practicioner, Sep 16 '19 at 03:19
Actually the only method you should be concerned with is `void OnConnected(string str)`. And handling UI properties is very well described in the duplicate. You cannot avoid invoking dispatcher. — FCin, Sep 16 '19 at 06:20

score 0 · Answer 1 · answered Sep 15 '19 at 18:04

0

Basically you can use the BackgroundWorker Control. here is a quick link. also here is an anther link which helps you.

also please read this:

How to: Make thread-safe calls to Windows Forms controls

answered Sep 15 '19 at 18:04

S.A.Parkhid

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score 0 · Accepted Answer · answered Sep 15 '19 at 18:36

H/T to S.A. Parkhid for this link.

Here is the solution: modify the method OnConnected to the following:

    public void OnConnected(string str)
    {
        if (textBox1.InvokeRequired)
        {
            var d = new TestEvent.clsConnect(OnConnected);
            textBox1.Invoke(d, new object[] { str });
        }
        else
        {
            textBox1.Text = str;
        }
    }

Sending an event from a thread to a UI

2 Answers2