This sample code is doing some long operation in a separate thread and try to update the UI from the new thread.

private void Form1_Load(object sender, EventArgs e)
{
    this.threadStart = new ThreadStart(LongProcess);
    this.thread = new Thread(this.threadStart);
    this.thread.Start();
}
private void LongProcess()
{
    for (int i = 0; i < 100; i++)
    {
        Thread.Sleep(100);
        //This will throw an exception like this
        //System.InvalidOperationException - Cross-thread operation not valid:
        //Control 'textBox1' accessed from a thread other than the thread it was created on.
        this.textBox1.Text = i.ToString();
    }
}

And here is the source code which will fix the issue with delegates and Control.Invoke method

private ThreadStart threadStart;
private Thread thread;
private delegate void UpdateTextDelegate(string text);
private void Form1_Load(object sender, EventArgs e)
{
    this.threadStart = new ThreadStart(LongProcess);
    this.thread = new Thread(this.threadStart);
    this.thread.Start();
}

private void LongProcess()
{
    for (int i = 0; i < 100; i++)
    {
        Thread.Sleep(100);
        //This will resolve the cross thread invalid operation exception.
        this.UpdateText(i.ToString());
    }
}

private void UpdateText(string text)
{
    //Checks whether the called required to invoke the "Invoke" method.
    if (this.textBox1.InvokeRequired)
    {
        UpdateTextDelegate updateTextDelegate = new UpdateTextDelegate(UpdateText);
        //Calling the invoke method of the control with the parameter.
        this.textBox1.Invoke(updateTextDelegate, new object[] { text });
    }
    else
    {
        this.textBox1.Text = text;
    }
}

Thanks to Praveen for helping me out. Happing Coding

EDIT: In case WPF applications, we don’t have InvokeRequired property available for Controls. Instead we need to use Dispatcher.CheckAccess() method. We can rewrite the UpdateText method like the following

private void UpdateText(string text)
{
if (this.textbox1.Dispatcher.CheckAccess())
{
    this.textbox1.Content = text;
}
else
{
UpdateTextDelegate updateTextHandler = new UpdateTextDelegate(UpdateText);
this.textbox1.Dispatcher.Invoke(updateTextHandler, new object[] { text });
}
}

Thanks to Sivadas for his valueable comments.