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How to: Make Thread-Safe Calls to Windows Forms Controls

Updated: September 2008

If you use multithreading to improve the performance of your Windows Forms applications, you must make sure that you make calls to your controls in a thread-safe way.

Access to Windows Forms controls is not inherently thread safe. If you have two or more threads manipulating the state of a control, it is possible to force the control into an inconsistent state. Other thread-related bugs are possible, such as race conditions and deadlocks. It is important to make sure that access to your controls is performed in a thread-safe way.

It is unsafe to call a control from a thread other than the one that created the control without using the Invoke method. The following is an example of a call that is not thread safe.

// This event handler creates a background thread that  
// attempts to set a Windows Forms control property  
// directly. 
private void setTextUnsafeBtn_Click
    (object sender, EventArgs e)
{
    // Create a background thread and start it. 
    this.demoThread =
        new Thread(new ThreadStart(this.ThreadProcUnsafe));
    this.demoThread.Start();

    // Continue in the main thread.  Set a textbox value that 
    // would be overwritten by demoThread if it succeeded. 
    // This value will appear immediately, then two seconds  
    // later the background thread will try to make its 
    // change to the textbox.
    textBox1.Text = "Written by the main thread.";
}

// This method is executed on the worker thread. It attempts 
// to access the TextBox control directly, which is not safe. 
private void ThreadProcUnsafe()
{
    // Wait two seconds to simulate some background work 
    // being done.
    Thread.Sleep(2000);

    this.textBox1.Text = 
        "Written unsafely by the background thread.";
}

The .NET Framework helps you detect when you are accessing your controls in a manner that is not thread safe. When you are running your application in the debugger, and a thread other than the one which created a control tries to call that control, the debugger raises an InvalidOperationException with the message, "Control control name accessed from a thread other than the thread it was created on."

This exception occurs reliably during debugging and, under some circumstances, at run time. You might see this exception when you debug applications that you wrote with the .NET Framework prior to the .NET Framework version 2.0. You are strongly advised to fix this problem when you see it, but you can disable it by setting the CheckForIllegalCrossThreadCalls property to false. This causes your control to run like it would run under Visual Studio .NET 2003 and the .NET Framework 1.1.

NoteNote:

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. For more information about using ActiveX controls with Windows Forms, see Windows Forms and Unmanaged Applications. If you are using Visual Studio, you can prevent this exception by disabling the Visual Studio hosting process. How to: Disable the Hosting Process
How to: Disable the Hosting Process
How to: Disable the Hosting Process
How to: Disable the Hosting Process
How to: Disable the Hosting Process

To make a thread-safe call to a Windows Forms control

  1. Query the control's InvokeRequired property.

  2. If InvokeRequired returns true, call Invoke with a delegate that makes the actual call to the control.

  3. If InvokeRequired returns false, call the control directly.

In the following code example, a thread-safe call is implemented in the ThreadProcSafe method, which is executed by the background thread. If the TextBox control's InvokeRequired returns true, the ThreadProcSafe method creates an instance of SetTextCallback and passes that to the form's Invoke method. This causes the SetText method to be called on the thread that created the TextBox control, and in this thread context the Text property is set directly.

// This event handler creates a thread that calls a  
// Windows Forms control in a thread-safe way. 
private void setTextSafeBtn_Click(
    object sender,
    EventArgs e)
{
    // Create a background thread and start it. 
    this.demoThread =
        new Thread(new ThreadStart(this.ThreadProcSafe));
    this.demoThread.Start();

    // Continue in the main thread.  Set a textbox value 
    // that will be overwritten by demoThread.
    textBox1.Text = "Written by the main thread.";
}

// If the calling thread is different from the thread that 
// created the TextBox control, this method passes in the 
// the SetText method to the SetTextCallback delegate and  
// passes in the delegate to the Invoke method. 
private void ThreadProcSafe()
{
    // Wait two seconds to simulate some background work 
    // being done.
    Thread.Sleep(2000);

    string text = "Written by the background thread.";
    // Check if this method is running on a different thread 
    // than the thread that created the control. 
    if (this.textBox1.InvokeRequired)
    {
        // It's on a different thread, so use Invoke.
        SetTextCallback d = new SetTextCallback(SetText);
        this.Invoke
            (d, new object[] { text + " (Invoke)" });
    }
    else
    {
        // It's on the same thread, no need for Invoke 
        this.textBox1.Text = text + " (No Invoke)";
    }
}
// This method is passed in to the SetTextCallBack delegate 
// to set the Text property of textBox1. 
private void SetText(string text)
{
    this.textBox1.Text = text;
}

The preferred way to implement multithreading in your application is to use the BackgroundWorker component. The BackgroundWorker component uses an event-driven model for multithreading. The background thread runs your DoWork event handler, and the thread that creates your controls runs your ProgressChanged and RunWorkerCompleted event handlers. You can call your controls from your ProgressChanged and RunWorkerCompleted event handlers.

To make thread-safe calls by using BackgroundWorker

  1. Create a method to do the work that you want done in the background thread. Do not call controls created by the main thread in this method.

  2. Create a method to report the results of your background work after it finishes. You can call controls created by the main thread in this method.

  3. Bind the method created in step 1 to the DoWork event of an instance of BackgroundWorker, and bind the method created in step 2 to the same instance’s RunWorkerCompleted event.

  4. To start the background thread, call the RunWorkerAsync method of the BackgroundWorker instance.

In the following code example, the DoWork event handler uses Sleep to simulate work that takes some time. It does not call the form’s TextBox control. The TextBox control's Text property is set directly in the RunWorkerCompleted event handler.

// This BackgroundWorker is used to demonstrate the  
// preferred way of performing asynchronous operations. 
private BackgroundWorker backgroundWorker1;
// This method starts BackgroundWorker by calling  
// RunWorkerAsync.  The Text property of the TextBox control 
// is set by a method running in the main thread 
// when BackgroundWorker raises the RunWorkerCompleted event. 
private void setTextBackgroundWorkerBtn_Click(
    object sender,
    EventArgs e)
{
    this.backgroundWorker1.DoWork += new DoWorkEventHandler(backgroundWorker1_DoWork);
    this.backgroundWorker1.RunWorkerCompleted += new System.ComponentModel.RunWorkerCompletedEventHandler(this.backgroundWorker1_RunWorkerCompleted);
    this.backgroundWorker1.RunWorkerAsync();

    // Continue in the main thread.
    textBox1.Text = "Written by the main thread.";
}

// This method does the work you want done in the background. 
void backgroundWorker1_DoWork (object sender, DoWorkEventArgs e)
{
    // Wait two seconds to simulate some background work 
    // being done.
    Thread.Sleep(2000);

    // You could use the same technique as in the  
    // ThreadProcSafe method to set textBox1.Text here, but  
    // the preferred method is to do it from the Completed  
    // event handler which runs in the same thread as the one 
    // that created the control.
}

// This method is called by BackgroundWorker's  
// RunWorkerCompleted event.  Because it runs in the 
// main thread, it can safely set textBox1.Text. 
private void backgroundWorker1_RunWorkerCompleted(
    object sender,
    RunWorkerCompletedEventArgs e)
{
    this.textBox1.Text =
        "Written by the main thread after the background thread completed.";
}

You can also report the progress of a background task by using the ProgressChanged event. For an example that incorporates that event, see BackgroundWorker.

The following code example is a complete Windows Forms application that consists of a form with three buttons and one text box. The first button demonstrates unsafe cross-thread access, the second button demonstrates safe access by using Invoke, and the third button demonstrates safe access by using BackgroundWorker.

NoteNote:

For instructions on how to run the example, see How to: Compile and Run a Complete Windows Forms Code Example Using Visual Studio. This example requires references to the System.Drawing and System.Windows.Forms assemblies.

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

namespace CrossThreadDemo
{
    public class Form1 : Form
    {
        // This delegate enables asynchronous calls for setting 
        // the text property on a TextBox control. 
        delegate void SetTextCallback(string text);

        // This thread is used to demonstrate both thread-safe and 
        // unsafe ways to call a Windows Forms control. 
        private Thread demoThread = null;

        // This BackgroundWorker is used to demonstrate the  
        // preferred way of performing asynchronous operations. 
        private BackgroundWorker backgroundWorker1;

        private TextBox textBox1;
        private Button setTextUnsafeBtn;
        private Button setTextSafeBtn;
        private Button setTextBackgroundWorkerBtn;

        private System.ComponentModel.IContainer components = null;

        public Form1()
        {
            InitializeComponent();
        }

        protected override void Dispose(bool disposing)
        {
            if (disposing && (components != null))
            {
                components.Dispose();
            }
            base.Dispose(disposing);
        }

        // This event handler creates a background thread that  
        // attempts to set a Windows Forms control property  
        // directly. 
        private void setTextUnsafeBtn_Click
            (object sender, EventArgs e)
        {
            // Create a background thread and start it. 
            this.demoThread =
                new Thread(new ThreadStart(this.ThreadProcUnsafe));
            this.demoThread.Start();

            // Continue in the main thread.  Set a textbox value that 
            // would be overwritten by demoThread if it succeeded. 
            // This value will appear immediately, then two seconds  
            // later the background thread will try to make its 
            // change to the textbox.
            textBox1.Text = "Written by the main thread.";
        }

        // This method is executed on the worker thread. It attempts 
        // to access the TextBox control directly, which is not safe. 
        private void ThreadProcUnsafe()
        {
            // Wait two seconds to simulate some background work 
            // being done.
            Thread.Sleep(2000);

            this.textBox1.Text = 
                "Written unsafely by the background thread.";
        }

        // This event handler creates a thread that calls a  
        // Windows Forms control in a thread-safe way. 
        private void setTextSafeBtn_Click(
            object sender,
            EventArgs e)
        {
            // Create a background thread and start it. 
            this.demoThread =
                new Thread(new ThreadStart(this.ThreadProcSafe));
            this.demoThread.Start();

            // Continue in the main thread.  Set a textbox value 
            // that will be overwritten by demoThread.
            textBox1.Text = "Written by the main thread.";
        }

        // If the calling thread is different from the thread that 
        // created the TextBox control, this method passes in the 
        // the SetText method to the SetTextCallback delegate and  
        // passes in the delegate to the Invoke method. 
        private void ThreadProcSafe()
        {
            // Wait two seconds to simulate some background work 
            // being done.
            Thread.Sleep(2000);

            string text = "Written by the background thread.";
            // Check if this method is running on a different thread 
            // than the thread that created the control. 
            if (this.textBox1.InvokeRequired)
            {
                // It's on a different thread, so use Invoke.
                SetTextCallback d = new SetTextCallback(SetText);
                this.Invoke
                    (d, new object[] { text + " (Invoke)" });
            }
            else
            {
                // It's on the same thread, no need for Invoke 
                this.textBox1.Text = text + " (No Invoke)";
            }
        }

        // This method is passed in to the SetTextCallBack delegate 
        // to set the Text property of textBox1. 
        private void SetText(string text)
        {
            this.textBox1.Text = text;
        }

        // This method starts BackgroundWorker by calling  
        // RunWorkerAsync.  The Text property of the TextBox control 
        // is set by a method running in the main thread 
        // when BackgroundWorker raises the RunWorkerCompleted event. 
        private void setTextBackgroundWorkerBtn_Click(
            object sender,
            EventArgs e)
        {
            this.backgroundWorker1.DoWork += new DoWorkEventHandler(backgroundWorker1_DoWork);
            this.backgroundWorker1.RunWorkerCompleted += new System.ComponentModel.RunWorkerCompletedEventHandler(this.backgroundWorker1_RunWorkerCompleted);
            this.backgroundWorker1.RunWorkerAsync();

            // Continue in the main thread.
            textBox1.Text = "Written by the main thread.";
        }

        // This method does the work you want done in the background. 
        void backgroundWorker1_DoWork (object sender, DoWorkEventArgs e)
        {
            // Wait two seconds to simulate some background work 
            // being done.
            Thread.Sleep(2000);

            // You could use the same technique as in the  
            // ThreadProcSafe method to set textBox1.Text here, but  
            // the preferred method is to do it from the Completed  
            // event handler which runs in the same thread as the one 
            // that created the control.
        }

        // This method is called by BackgroundWorker's  
        // RunWorkerCompleted event.  Because it runs in the 
        // main thread, it can safely set textBox1.Text. 
        private void backgroundWorker1_RunWorkerCompleted(
            object sender,
            RunWorkerCompletedEventArgs e)
        {
            this.textBox1.Text =
                "Written by the main thread after the background thread completed.";
        }

        #region Windows Form Designer generated code

        private void InitializeComponent()
        {
            this.textBox1 = new System.Windows.Forms.TextBox();
            this.setTextUnsafeBtn = new System.Windows.Forms.Button();
            this.setTextSafeBtn = new System.Windows.Forms.Button();
            this.setTextBackgroundWorkerBtn = new System.Windows.Forms.Button();
            this.backgroundWorker1 = new System.ComponentModel.BackgroundWorker();
            this.SuspendLayout();
            //  
            // textBox1 
            //  
            this.textBox1.Location = new System.Drawing.Point(12, 12);
            this.textBox1.Name = "textBox1";
            this.textBox1.Size = new System.Drawing.Size(360, 20);
            this.textBox1.TabIndex = 0;
            //  
            // setTextUnsafeBtn 
            //  
            this.setTextUnsafeBtn.Location = new System.Drawing.Point(15, 55);
            this.setTextUnsafeBtn.Name = "setTextUnsafeBtn";
            this.setTextUnsafeBtn.TabIndex = 1;
            this.setTextUnsafeBtn.Text = "Unsafe Call";
            this.setTextUnsafeBtn.Click += new System.EventHandler(this.setTextUnsafeBtn_Click);
            //  
            // setTextSafeBtn 
            //  
            this.setTextSafeBtn.Location = new System.Drawing.Point(96, 55);
            this.setTextSafeBtn.Name = "setTextSafeBtn";
            this.setTextSafeBtn.TabIndex = 2;
            this.setTextSafeBtn.Text = "Safe Call";
            this.setTextSafeBtn.Click += new System.EventHandler(this.setTextSafeBtn_Click);
            //  
            // setTextBackgroundWorkerBtn 
            //  
            this.setTextBackgroundWorkerBtn.Location = new System.Drawing.Point(177, 55);
            this.setTextBackgroundWorkerBtn.Name = "setTextBackgroundWorkerBtn";
            this.setTextBackgroundWorkerBtn.TabIndex = 3;
            this.setTextBackgroundWorkerBtn.Text = "Safe BW Call";
            this.setTextBackgroundWorkerBtn.Click += new System.EventHandler(this.setTextBackgroundWorkerBtn_Click);
            //  
            // Form1 
            //  
            this.ClientSize = new System.Drawing.Size(388, 96);
            this.Controls.Add(this.setTextBackgroundWorkerBtn);
            this.Controls.Add(this.setTextSafeBtn);
            this.Controls.Add(this.setTextUnsafeBtn);
            this.Controls.Add(this.textBox1);
            this.Name = "Form1";
            this.Text = "Form1";
            this.ResumeLayout(false);
            this.PerformLayout();

        }

        #endregion


        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.Run(new Form1());
        }

    }
}

When you run the application and click the Unsafe Call button, you immediately see "Written by the main thread" in the text box. Two seconds later, when the unsafe call is attempted, the Visual Studio debugger indicates that an exception occurred. The debugger stops at the line in the background thread that attempted to write directly to the text box. You will have to restart the application to test the other two buttons. When you click the Safe Call button, "Written by the main thread" appears in the text box. Two seconds later, the text box is set to "Written by the background thread (Invoke)", which indicates that the Invoke method was called. When you click the Safe BW Call button, "Written by the main thread" appears in the text box. Two seconds later, the text box is set to "Written by the main thread after the background thread completed", which indicates that the handler for the RunWorkerCompleted event of BackgroundWorker was called.

Caution noteCaution:

When you use multithreading of any sort, your code can be exposed to very serious and complex bugs. For more information, see Managed Threading Best Practices before you implement any solution that uses multithreading.

Date

History

Reason

September 2008

Updated topic with additional explanatory text.

Customer feedback.

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