xamarin EventHandler is always null

Question

I have a code which should notify a delegate in one of service classes on receiving an event:

    public class TestClass : ParentClass
    {
        public event EventHandler<string> MyDelegate;

        public override void OnAction(Context context, Intent intent)
        {
            var handler = MyDelegate;
            if (handler != null)
            {
                handler(this, "test");
            }
        }
    }

I instantiate it by:

    private TestClass mytest= new TestClass ();

And then assign it in one of the functions:

    mytest.MyDelegate+= (sender, info) => {
    };

The delegate never gets called. I have stepped through with debugger and I see that delegate is being assigned, but the check inside a class is always null... Cant figure out whats the problem...

Answer 1

Sounds like an execution order issue. What could be happening is that OnAction within TestClass is being called before the delegate hookup. Try the following:

public class TestClass : ParentClass
{
    public event EventHandler<string> MyDelegate;

    public class TestClass(Action<string> myAction)
    {
      MyDelegate += myAction;
    }

    public override void OnAction(Context context, Intent intent)
    {
        var handler = MyDelegate;
        if (handler != null)
        {
            handler(this, "test");
        }
    }
}

Just pass the delegate through the constructor, this should make sure its hooked up before any calls to OnAction()

You can pass through the handler in a couple ways:

1.) As an anonymous method:

private TestClass mytest= new TestClass ((sender, info) => { Console.WriteLine("Event Attached!") });

2.) Pass in the method group:

public class MyEventHandler(object sender, string e)
{
  Console.WriteLine("Event Attached!");
}

private TestClass mytest= new TestClass(MyEventHandler);

I generally recommend the second way as it allows you to unhook the handler and do clean up once you are done with it:

myTest.MyDelegate -= MyEventHandler;