Hello everyone,<\/p>\n
Recently I was asked in class how to solve the whole “Unable to access control created on another thread” exception.
\nThe teachers apperantly didn’t know the solution so my classmates turned to me.<\/p>\n
It’s pretty simple actually. <\/p>\n Let’s say we have 2 threads.<\/p>\n The main thread being our application when it starts, and a worker thread that’s created when we press a button. Now, if we do not delegate this task to the main thread when it’s being called from the worker thread, it will raise an exception saying it can’t access controls created by \/ on a different thread.<\/p>\n So the only thing we have to do, is create a delegate for this:<\/p>\n VB.NET:<\/strong><\/p>\n How to declare a delegate in VB.NET<\/em><\/p>\n C#.NET<\/strong><\/p>\n How to declare a delegate in C#.NET<\/em><\/p>\n That’s about it when it comes down to actually creating<\/strong> the delegates. So we will create a new thread when we click a button, and that thread will execute a method called “DoIncrement()” VB.NET:<\/strong><\/p>\n The DoIncrement() procedure in VB.NET<\/em><\/p>\n C#.NET<\/strong><\/p>\n The DoIncrement() procedure in C#.NET<\/em><\/p>\n Now, in the “ShowValue()” method we will call upon the delegate we’ve declared. VB.NET:<\/strong><\/p>\n The ShowValue() procedure in VB.NET<\/em><\/p>\n C#.NET:<\/strong><\/p>\n The ShowValue() procedure in C#.NET<\/em><\/p>\n Here is the complete source code for the VB.NET and C#.NET projects. VB.NET<\/strong><\/p>\n The source code in VB.NET<\/em><\/p>\n C#.NET<\/strong><\/p>\n The source code in C#<\/em><\/p>\n There you go, access controls from other threads ;P<\/p>\n If you have any questions regarding this topic, feel free to ask and I’ll do my best to answer them.<\/p>\n Happy coding!<\/p>\n – Dirk<\/p>\n","protected":false},"excerpt":{"rendered":" Hello everyone, Recently I was asked in class how to solve the whole “Unable to access control created on another thread” exception. The teachers apperantly didn’t know the solution so my classmates turned to me. It’s pretty simple actually. What we basicly have to do, is delegate the task.<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":[],"categories":[38],"tags":[],"_links":{"self":[{"href":"https:\/\/dirk.schuermans.me\/index.php?rest_route=\/wp\/v2\/posts\/455"}],"collection":[{"href":"https:\/\/dirk.schuermans.me\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/dirk.schuermans.me\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/dirk.schuermans.me\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/dirk.schuermans.me\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=455"}],"version-history":[{"count":6,"href":"https:\/\/dirk.schuermans.me\/index.php?rest_route=\/wp\/v2\/posts\/455\/revisions"}],"predecessor-version":[{"id":776,"href":"https:\/\/dirk.schuermans.me\/index.php?rest_route=\/wp\/v2\/posts\/455\/revisions\/776"}],"wp:attachment":[{"href":"https:\/\/dirk.schuermans.me\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=455"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/dirk.schuermans.me\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=455"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/dirk.schuermans.me\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=455"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}
\nWhat we basicly have to do, is delegate<\/strong> the task.<\/p>\n
\nWhen the worker thread has been started, we want it to update a TextBox value and increment it by 1.<\/p>\n'Delegate\r\nPrivate Delegate Sub MyDelegate()<\/pre>\n
\/\/ Delegate\r\npublic delegate void MyDelegate();<\/pre>\n
\nNow it’s time to use them.<\/p>\n
\nThis method will loop untill our counter is 10 and it will sleep for 1000 miliseconds (= 1 second) after it has incremented the value.
\nOnce the counter has been incremented, a method called “ShowValue()” will be called to display the current value in a TextBox<\/p>\nPrivate Sub DoIncrement()\r\n\tWhile (Me.m_nCounter < 10)\r\n\t\t'Increment the counter\r\n\t\tMe.m_nCounter += 1\r\n\t\tMe.ShowValue()\r\n\r\n\t\t'Let the worker thread sleep\r\n\t\tThread.Sleep(1000)\r\n\tEnd While\r\n\r\n\tMe.m_nCounter = 0\r\n\tMessageBox.Show(\"Worker thread is done!\")\r\nEnd Sub<\/pre>\n
\/\/\/\r\n\r\n\/\/\/ Increments the counter by 1\r\n\/\/\/ \r\n\r\nprivate void DoIncrement()\r\n{\r\n\twhile (m_nCounter < 10)\r\n\t{\r\n\t\t\/\/ Increment the counter\r\n\t\tthis.m_nCounter++;\r\n\t\tthis.ShowValue();\r\n\r\n\t\t\/\/ Pause the worker thread for 1 second\r\n\t\tThread.Sleep(1000);\r\n\t}\r\n\r\n\tthis.m_nCounter = 0;\r\n\tMessageBox.Show(\"Worker thread has finished!\");\r\n}<\/pre>\n
\nWhat we’ll do is check if the TextBox we’re trying to show the value in belongs to another thread and if it does, delegate the task “ShowValue” to the thread that owns the control.<\/p>\nPrivate Sub ShowValue()\r\n\tIf (Me.txtValue.InvokeRequired) Then\r\n\t\t'Declare a new instance of our delegate\r\n\t\tDim Show As MyDelegate\r\n\r\n\t\t'Setup the delegate\r\n\t\tShow = AddressOf ShowValue\r\n\r\n\t\t'Invoke the delegate for the control\r\n\t\tMe.txtValue.Invoke(Show)\r\n\tElse\r\n\t\t'Update the value of the textbox\r\n\t\ttxtValue.Text = m_nCounter.ToString()\r\n\tEnd If\r\nEnd Sub<\/pre>\n
\/\/\/\r\n\r\n\/\/\/ Shows the current value of m_nCounter in txtValue\r\n\/\/\/ \r\n\r\nprivate void ShowValue()\r\n{\r\n\t\/\/ Check to which thread the control belongs\r\n\tif (this.txtValue.InvokeRequired)\r\n\t{\r\n\t\t\/\/ Fire the delegate\r\n\t\tthis.txtValue.Invoke(new MyDelegate(ShowValue));\r\n\t}\r\n\telse\r\n\t{\r\n\t\t\/\/ Show the current value\r\n\t\tthis.txtValue.Text = this.m_nCounter.ToString();\r\n\t}\r\n}<\/pre>\n
\nAll you need is a small form with 1 button called btnStart<\/strong> and a TextBox called txtValue<\/strong><\/p>\nImports System.Threading\r\n\r\nPublic Class Form1\r\n\r\n 'Delegate\r\n Private Delegate Sub MyDelegate()\r\n\r\n 'Variables\r\n Private m_nCounter As Integer\r\n Private m_trdCounter As Thread\r\n\r\n '''\r\n\r\n ''' Occurs when the user clicks the start button\r\n ''' \r\n\r\n '''\r\nSystem.Object : The object calling this procedure\r\n '''\r\nSystem.EventArgs : The event arguments\r\n ''' \r\n Private Sub btnStart_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnStart.Click\r\n 'Setup the thread\r\n Me.m_trdCounter = New Thread(AddressOf Me.DoIncrement)\r\n Me.m_trdCounter.IsBackground = True\r\n\r\n 'Start the new thread\r\n Me.m_trdCounter.Start()\r\n End Sub\r\n\r\n '''\r\n\r\n ''' Increment the value of m_nCounter by 1 and call ShowValue()\r\n ''' \r\n\r\n ''' \r\n Private Sub DoIncrement()\r\n While (Me.m_nCounter < 10)\r\n 'Increment the counter\r\n Me.m_nCounter += 1\r\n Me.ShowValue()\r\n\r\n 'Let the worker thread sleep\r\n Thread.Sleep(1000)\r\n End While\r\n\r\n Me.m_nCounter = 0\r\n MessageBox.Show(\"Worker thread is done!\")\r\n End Sub\r\n\r\n '''\r\n\r\n ''' Show the current value of m_nCounter in the TextBox\r\n ''' \r\n\r\n ''' \r\n Private Sub ShowValue()\r\n If (Me.txtValue.InvokeRequired) Then\r\n 'Declare a new instance of our delegate\r\n Dim Show As MyDelegate\r\n\r\n 'Setup the delegate\r\n Show = AddressOf ShowValue\r\n\r\n 'Invoke the delegate for the control\r\n Me.txtValue.Invoke(Show)\r\n Else\r\n 'Update the value of the textbox\r\n txtValue.Text = m_nCounter.ToString()\r\n End If\r\n End Sub\r\nEnd Class<\/pre>\n
using System;\r\nusing System.Collections.Generic;\r\nusing System.ComponentModel;\r\nusing System.Data;\r\nusing System.Drawing;\r\nusing System.Linq;\r\nusing System.Text;\r\nusing System.Windows.Forms;\r\nusing System.Threading;\r\n\r\nnamespace Crossthreading_Example\r\n{\r\n \/\/ Delegate\r\n public delegate void MyDelegate();\r\n\r\n public partial class Form1 : Form\r\n {\r\n \/\/ Variables\r\n private int m_nCounter;\r\n private Thread m_trdCounter;\r\n\r\n \/\/\/\r\n\r\n \/\/\/ Default constructor\r\n \/\/\/ \r\n\r\n public Form1()\r\n {\r\n InitializeComponent();\r\n\r\n \/\/ Initialize\r\n m_nCounter = 0;\r\n }\r\n\r\n \/\/\/\r\n\r\n \/\/\/ Occurs when the user clicks the start button\r\n \/\/\/ \r\n\r\n \/\/\/\r\nSystem.Object : The object calling this method\r\n \/\/\/\r\nSystem.EventArgs : The event arguments\r\n private void btnStart_Click(object sender, EventArgs e)\r\n {\r\n \/\/ Setup a new thread\r\n this.m_trdCounter = new Thread(DoIncrement);\r\n this.m_trdCounter.IsBackground = true;\r\n\r\n \/\/ Start the thread\r\n this.m_trdCounter.Start();\r\n }\r\n\r\n \/\/\/\r\n\r\n \/\/\/ Increments the counter by 1\r\n \/\/\/ \r\n\r\n private void DoIncrement()\r\n {\r\n while (m_nCounter < 10)\r\n {\r\n \/\/ Increment the counter\r\n this.m_nCounter++;\r\n this.ShowValue();\r\n\r\n \/\/ Pause the worker thread for 1 second\r\n Thread.Sleep(1000);\r\n }\r\n\r\n this.m_nCounter = 0;\r\n MessageBox.Show(\"Worker thread has finished!\");\r\n }\r\n\r\n \/\/\/\r\n\r\n \/\/\/ Shows the current value of m_nCounter in txtValue\r\n \/\/\/ \r\n\r\n private void ShowValue()\r\n {\r\n \/\/ Check to which thread the control belongs\r\n if (this.txtValue.InvokeRequired)\r\n {\r\n \/\/ Fire the delegate\r\n this.txtValue.Invoke(new MyDelegate(ShowValue));\r\n }\r\n else\r\n {\r\n \/\/ Show the current value\r\n this.txtValue.Text = this.m_nCounter.ToString();\r\n }\r\n }\r\n }\r\n}<\/pre>\n