How to implement an event which can be canceled?

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.

Answer 2

Your BuildStartEventArgs are redundant, the framework already offers the CancelEventArgs class – consider using it.