Question

Help me to implement an event, which handler can cancel it.

public class BuildStartEventArgs : EventArgs {     public bool Cancel { get; set; } }  class Foo {     public event EventHandler<BuildStartEventArgs> BuildStart;      private void Bar()     {         // build started         OnBuildStart(new BuildStartEventArgs());         // how to catch cancellation?     }      private void OnBuildStart(BuildStartEventArgs e)     {         if (this.BuildStart != null)         {             this.BuildStart(this, e);         }     } } 

Answer 1

You need to modify this code:

private void Bar() {     // build started     OnBuildStart(new BuildStartEventArgs());     // how to catch cancellation? } 

to something like this:

private void Bar() {     var e = new BuildStartEventArgs();     OnBuildStart(e);     if (!e.Cancel) {       // Do build     } } 

Classes in .NET have reference semantics, so you can see any changes made to the object the parameter of the event references.