How do I setup an event handler correctly?

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I am having a problem getting the hang of event handlers. This is a sample of the code I
am using, and it causes an error. Am I missing something? This is not the complete code
file, but only the part that is causing the problem. There are also additional handlers
in the file, but they are all setup the same way, and this is the frist one in the list,
which when it builds is the first one attempted(I think). Can anybody point out what I
am doing wrong?

C#

using System;
using System.Reflection;

public class Control 
{
        private EventInfo mouseEvent;
        public event EventHandler MouseLeftPress = delegate { };
        private delegate void MouseLeftPressHandler(object sender, EventArgs e);
        public void MouseLeftPressMethod(object sender, EventArgs e)
        {
            _IsLeftClicked = true;
            MousePressed();
        }
        public virtual void MousePressed()
        {
            if (!_IsDisabled)
            {
                _State = ControlState.pressed;
                _IsReleased = false;
                _DrawRectangle.X = _StartPoints[(int)ControlState.pressed].X;
                _DrawRectangle.Y = _StartPoints[(int)ControlState.pressed].Y;
            }
        }

	public Control()
	{
	        MethodInfo mouseLeftPressMethod = this.GetType().GetMethod("MouseLeftPressed");
	        mouseLeftPressEventHandler = new EventHandler(MouseLeftPressMethod);
		mouseEvent.AddEventHandler(null, mouseLeftPressEventHandler);
	}
}

causes this error:
An unhandled exception of type 'System.NullReferenceException' occurred in Suduko2015.exe

Additional information: Object reference not set to an instance of an object.

What I have tried:

I have tried changing the "null" to "this"; but maybe thats not right? HELP!

Posted 15-Jun-19 4:32am

Jester58

Updated 15-Jun-19 11:30am

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BillWoodruff 15-Jun-19 17:14pm

This doesn't appear to be a C# winForm app: please add tags to indicate the FrameWork and.or IDE used ... WPF ? Unity ?

Also why are you injecting EventHandlers using reflection into a run-time instance in the Class 'ctor ? That's very odd.

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OriginalGriff · Answer 1 · 2019-06-15T05:02:00

Adding an event is simple: A Simple Code Snippet to Add an Event[^] even automates it for you!

As to why your code gives you a null reference, we can't really say - we don't have access to your code while it's running so we can't tell where the null is being found.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterdays shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, VS will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, VS will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!

BillWoodruff · Answer 2 · 2019-06-15T11:31:00

C#

MethodInfo mouseLeftPressMethod = this.GetType().GetMethod("MouseLeftPressed");

'MouseLeftPressed is an EventInfo, not a Method ! Executing this will result in a null value for 'mouseLeftPressMethod. Use "MousePressed" instead.

mouseEvent.AddEventHandler(null, mouseLeftPressEventHandler);

You never instantiate 'mouseEvent: that's why you get a null reference exception.

Also why are you injecting EventHandlers using reflection into a run-time instance in the Class 'ctor ? That's very odd.

Please add tags to indicate the FrameWork and.or IDE used ... WinForm ?WPF ? Unity ?

How do I setup an event handler correctly?

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