Accessing controls from another thread in C#

When we try to access a control from another thread, you may see an exception, saying 'InvalidOperationException'. Like below

This is raised because a control was accessed without invoking.

One method is to check if that particular object required to be invoked.

If it needs to be invoked, Invoke it else just access it.

A simple way to do this is described in this post.

First Add these references in your code

using System.Threading;

Declare two delegates as given below

private delegate void DSetText(string txt); private delegate void DSetEnable(Control control, bool enable);

Define the function to change the text of a control as below

private void SetText(string text) {     //check if the label needs to be invoked     if (lblText.InvokeRequired)     {        //create an object of the delegate. The argument is the method (SetText) itself        DSetText d = new DSetText(SetText);        //now lets invoke it. pass the arguments of the method as an object array        this.Invoke(d, new object[] { text });      }      else      {         //no invoke required so lets set the text         lblText.Text = text;         //sometimes the label needs to be refreshed to view the changed text         lblText.Refresh();               } }

Define the function to enable/disable a control as below

private void SetEnable(Control control,bool enable) {    //check if the control needs to be invoked    if (control.InvokeRequired)    {       //create an object of the delegate the argument is the method (SetText) itself       DSetEnable d = new DSetEnable(SetEnable);       //now lets invoke it. pass the arguments of the method as an object array       this.Invoke(d, new object[] { control,enable });    }    else    {       //no invoke required so lets enable/disable the control       control.Enabled = enable;    } }

Declare an object of the Thread. This is the thread where a time consuming work executes 

Thread myThread = null;

Declare a variable to stop execution of the thread. When this value is True then thread stops  execution.

bool StopRequested = false;

Define a function to execute in the thread (myThread)

public void StartRunning() {    int count = 0;    //disable the start button    SetEnable(btnStart, false);    //enable stop button    SetEnable(btnStop, true);               //execute until StopRequested is true. StopRequested is set when btnStop is clicked.    while(!StopRequested)    {      //set the count as label's text      SetText(count.ToString());      //lets wait for some time      Thread.Sleep(100);      count++;    }    //enable the start button    SetEnable(btnStart, true);    //disable stop button    SetEnable(btnStop, false); }

Now on the Click event of the button( Button to start the thread) add the below code

StopRequested = false; myThread = new Thread(new ThreadStart(StartRunning)); myThread.Start();

Now on the Click event of the other button( Button to stop the thread execution)

StopRequested = true;

The overall code is given below

using System;  using System.Collections.Generic;  using System.ComponentModel;  using System.Data;  using System.Drawing;  using System.Linq;  using System.Text;  using System.Windows.Forms;  using System.Threading;  namespace ThreadSafeApp  {     public partial class Form1 : Form     {         public Form1()         {             InitializeComponent();         }         //Delegate to set the text         private delegate void DSetText(string txt);         //delegate to enable/disable a control         private delegate void DSetEnable(Control control, bool enable);         //Method to set the text of label         private void SetText(string text)         {             //check if the label needs to be invoked             if (lblText.InvokeRequired)             {                      //create an object of the delegate. The argument is the method (SetText) itself                 DSetText d = new DSetText(SetText);                 //now lets invoke it. pass the arguments of the method as an object array                 this.Invoke(d, new object[] { text });             }             else             {                 //no invoke required so lets set the text                 lblText.Text = text;                 //sometimes the label needs to be refreshed to view the changed text                 lblText.Refresh();             }         }         private void SetEnable(Control control,bool enable)         {             //check if the control needs to be invoked             if (control.InvokeRequired)             {             //create an object of the delegate. The argument is the method (SetText) itself                 DSetEnable d = new DSetEnable(SetEnable);                 //now lets invoke it. pass the arguments of the method as an object array                 this.Invoke(d, new object[] { control,enable });             }             else             {                 //no invoke required so lets enable/disable the control                 control.Enabled = enable;             }         }         //thread         Thread myThread = null;                        bool StopRequested = false;                 public void StartRunning()         {             int count = 0;             //disable the start button             SetEnable(btnStart, false);             //enable stop button             SetEnable(btnStop, true);                         //execute until StopRequested is true.             //StopRequested is set when btnStop is clicked.             while(!StopRequested)            {                //set the count as label's text                SetText(count.ToString());                //lets wait for some time                Thread.Sleep(100);                count++;            }             //enable the start button             SetEnable(btnStart, true);             //disable stop button             SetEnable(btnStop, false);         }                 private void btnStart_Click(object sender, EventArgs e)         {             StopRequested = false;             myThread = new Thread(new ThreadStart(StartRunning));             myThread.Start();                   }         private void btnStop_Click(object sender, EventArgs e)         {             StopRequested = true;                  }     }  }