user be able to cancle the process

You can also use

private void Buttoncancel_Click(object sender, System.EventArgs e)
{
   System.Diagnostics.Process[] myProcesses;
   myProcesses =
      System.Diagnostics.Process.GetProcessesByName("ProcessName");
   foreach (System.Diagnostics.Process instance in myProcesses)
   {
      instance.CloseMainWindow();
   }
}

It depends on how you are doing it.
If you have the long operation running in the UI thread (ie, you haven't moved it to a separate thread already) then it will block the user interface from doing anything - so it can't respond to you mouse clicks or button presses. There is a way round that, by calling Application.DoEvents, but it is a very nasty solution, and can make response sluggish if you don't call it often enough, or make the processing much much longer if you call it too often.

A much better solution is to put the long operation in a separate thread - look at the BackgroundWorker class[^] - it's pretty simple, and the link includes a sample with a cancel button.

If you already have it if a separate task, then just cancel the operation!

Have a look at the BackgroundWorker Class[^]. You could execute your logic using the background worker and offer the user a possibility to cancel the execution by calling CancelAsync [^] in the button.