Add a control on a form, from another Thread.

Question

Hello to everyone.

This is my first post on this site, which I must say is, great!

Well here is my problem, and I truly hope someone will be able to help me.

I was trying to postpone adding controls to my main form, with a goal to speed up it's start time. Well I run in the following exception:

Cross-thread operation not valid: Control 'Form1' accessed from a thread other than the thread it was created on.

I tried to simply the problem on a smaller example but the problem stays. Here is my code:

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

namespace AddConrolFromAnotherThread {
    public partial class Form1 : Form {

        public Form1() {
            InitializeComponent();
        }


        private void AddButton() { 
            if(this.InvokeRequired){
                this.Invoke(new MethodInvoker(this.AddButton));
            }
            Random random = new Random(2);
            Thread.Sleep(20);
            Button button = new Button();
            button.Size = new Size(50,50);
            button.Location = 
                new Point(random.Next(this.Width),random.Next(this.Height));
                this.Controls.Add(button);
        }

        private void buttonStart_Click(object sender, EventArgs e) {
            Thread addControlThread = 
                new Thread(new ThreadStart(this.AddButton));
            addControlThread.Start();
        }
    }
}

I did use the Invoke method and did check if InvokeRequiered is true, but InvokeRequiered keep staying "true". I really don't understand that. At least I would expect StackOverflow exception, since this is a recursion call.

So, if anyone met the similar problem, please could you tell me what did I do wrong?

Thanx, Igor!

Answer 1

Use anonymous methods instead. Explanation is below.

If we have form like this:

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

    private void Form1_Load(object sender, EventArgs e)
    {
        Thread t = new Thread(new ThreadStart(Start));
        t.Start();
    }

    private void UpdateText()
    {
        button1.Text = "New Text";
    }

    void Start()
    {
        UpdateText();
    }
}

This will throw an exception.

Change UpdateText to:

private delegate void MyDelegate();

private void UpdateText()
{
    if (button1.InvokeRequired)
    {
       button1.Invoke(new MyDelegate(UpdateText));
    }
    button1.Text = "New Text";
}

or use anonymous method:

void Start() 
{
    this.Invoke((MyDelegate)delegate
    {
        UpdateText();
    });
}

private void UpdateText()
{
    button1.Text = "New Text";
}

Answer 2

The problem in your code is that your are adding two buttons.

Put the code after the if block in an else block.

private void AddButton() { 
        if(this.InvokeRequired){
            this.Invoke(new MethodInvoker(this.AddButton));
        }
        else {
           Random random = new Random(2);
           Thread.Sleep(20);
           Button button = new Button();
           button.Size = new Size(50,50);
           button.Location = new Point(random.Next(this.Width),random.Next(this.Height));
           this.Controls.Add(button);
        }
    }

Answer 3

It's very expensive to use a thread for just adding a button ! Use the ThreadPool instead.