Thread-Safe Calls Using Windows Form Controls in C#

using System.Threading;

Thread th = new Thread(new ThreadStart(this.ThreadProcUnsafe));
th.Start();
myTextBox.Text = "Written by the main thread.";

private void ThreadProcUnsafe()
{
    Thread.Sleep(2000);
    this.textBox1.Text = "Written unsafely by the background thread.";
}

using System;
using System.Threading;
using System.Windows.Forms;
namespace ThreadUnsafecalltowindowform
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private void UnsafeBtn_Click(object sender, EventArgs e)
        {
           Thread th = new Thread(new ThreadStart(this.ThreadProcUnsafe));
            th.Start();
            myTextBox1.Text = "Written by the main thread.";
        }
        private void ThreadProcUnsafe()
        {
            Thread.Sleep(2000);
            this.myTextBox1.Text = "Written unsafely by the background thread.";
        }
    }
}

If you are using ActiveX controls on a form, you may receive the cross-thread InvalidOperationException when you run under the debugger. When this occurs, the ActiveX control does not support multithreading.

The Standard solution of the problem

There is, of course, a mechanism to deal with this. Each Windows Forms control has the InvokeRequired property which returns false if the current thread is the message queue thread. And there is the Invoke method which makes it possible to enqueue a delegate complete with parameters into the message queue of the control.

Since the delegate is called directly from the message queue, no threading issues arise. But this style of programming can be quite tedious. Just to do something as simple as setting a text property or enabling/disabling a control, you have to define a separate method with a matching delegate.

code

A simple example of using this is to create a new form, add a textbox and a button to it. Call the textbox myTextBox.

At the top of the code file add another using statement for the threading library.

using System.Threading;

In the button's Click event place the following code.

Thread th = new Thread(new ThreadStart(this.ThreadProcUnsafe));
th.Start();
myTextBox.Text = "Written by the main thread.";

Now add this code to the form

private void ThreadProcSafe()
{
    Thread.Sleep(2000); 
    string text = "Written by the background thread.";
    if (this.myTextBox.InvokeRequired)
    {
        SetTextCallback d = new SetTextCallback(SetText);
        this.Invoke(d, new object[] { text + " (Invoke)" });
    }
    else
    {
        this.myTextBox.Text = text + " (No Invoke)";
    }
}

private void SetText(string text)
{
    this .myTextBox.Text=text;
}

Outside of the form's class add this declaration.

delegate void SetTextCallback(string text);

The complete form1.cs file are look like as

using System;
using System.Windows.Forms;
using System.Threading;
namespace ThreadSafeCallsToWindowForm
{
    delegate void SetTextCallback(string text);
    public partial class Form1 : Form
    { 
        public Form1()
        {
            InitializeComponent();
       }      
        private void SetText(string text)
        {
            this .myTextBox.Text=text;
        }
        private void Safebutton_Click(object sender, EventArgs e)
        {
            Thread th = new Thread(new ThreadStart(this.ThreadProcSafe));
            th.Start();
            myTextBox.Text = "Written by the main thread.";
        }
        private void ThreadProcSafe()
        {
           Thread.Sleep(2000);
             string text = "Written by the background thread.";
            if (this.myTextBox.InvokeRequired)
            {
                SetTextCallback d = new SetTextCallback(SetText);
                this.Invoke(d, new object[] { text + " (Invoke)" });
           }
            else
            {
                // It's on the same thread, no need for Invoke
                this.myTextBox.Text = text + " (No Invoke)";
            }
        } 
    }
}

Output

Run the application

Click the show button

After two seconds: