c# - TargetInvokationException in Application.Run(new Form1()); -

i trying iterate through loop pressing start button , stop pressing stop button. using await task.run(() => works in expected manner. when press start button again, targetinvokationexception in application.run(new form1());.

my code below

using system.threading; using system.threading.tasks; using system.windows.forms;  namespace cancellationtest {     public partial class form1 : form     {         private readonly synchronizationcontext synchronizationcontext;         private datetime previoustime = datetime.now;          cancellationtokensource cts = new cancellationtokensource();          public form1()         {             initializecomponent();             synchronizationcontext = synchronizationcontext.current;         }          private async void buttonclickhandlerasync(object sender, eventargs e)         {             button1.enabled = false;             var count = 0;              cancellationtoken token = cts.token;              await task.run(() =>             {                 try                 {                     (var = 0; <= 5000000; i++)                     {                         token.throwifcancellationrequested();                          updateui(i);                         count = i;                     }                 }                 catch (system.operationcanceledexception)                 {                     messagebox.show("canceled");                 }             }, token);              label1.text = @"counter " + count;             button1.enabled = true;         }          public void updateui(int value)         {             var timenow = datetime.now;              if ((datetime.now - previoustime).milliseconds <= 50) return;              synchronizationcontext.post(new sendorpostcallback(o =>             {                 label1.text = @"counter " + (int)o;             }), value);              previoustime = timenow;         }          private void button2_click(object sender, eventargs e)         {             cts.cancel();         }     } } 

can explain why happens , how resolve this.

can explain why happens

targetinvokationexception wrapper type exception , main information contained in innerexception property didn't show. looking @ code, operationcanceledexception , caused passing canceled token task.run.

in general should not reuse cancellationtokensource instance - see the general pattern implementing cooperative cancellation model in remarks section of documentation.

also should protect try/catch whole block, not task.run body. , initialize involved state members @ beginning, , necessary cleanup @ end.

so correct code this:

private datetime previoustime; private cancellationtokensource cts;  private async void buttonclickhandlerasync(object sender, eventargs e) {     button1.enabled = false;     var count = 0;     previoustime = datetime.now;     cts = new cancellationtokensource();     try     {         cancellationtoken token = cts.token;         await task.run(() =>         {             (var = 0; <= 5000000; i++)             {                 token.throwifcancellationrequested();                 updateui(i);                 count = i;             }         }, token);     }     catch (system.operationcanceledexception)     {         messagebox.show("canceled");     }         {         cts.dispose();         cts = null;     }     label1.text = @"counter " + count;     button1.enabled = true; } 

and make sure cancel button enabled when cts != null or check condition inside click handler in order avoid nre.