Question

I have two classes, and i have to make an event to communicate between these classes.

Class a
{
    public delegate void delegat(int a);
    public event delegat exit;
    ...
    private void a_FormClosed(object sender, FormClosedEventArgs e)
    {
         // My event named exit should run here, but I get exception!
        exit(100);
    }
}

Class b
{
    a instance=new a();
    a.exit+=new a.delegat(my_fun);
    ...
    private void my_fun(int x)
    {
        if(x==100)
        do_smth;
        ...
    }
}

But the thing is that i get exception: “object reference not set to an instance of an object”.
I can’t understand what Am I doing wrong? Where should I make a new instance of this?
Thanks for help!

Answer 1

You are trying to assign the exit event on the class itself and not the instance e.g.

a.exit += ...

Should be:

instance.exit += ...

You also aren’t checking whether your exit event has been assigned before attempting to fire it. There are other issues which you haven’t taken into consideration like race conditions.

Here is a general example of a relatively safe way of handling events e.g.

public class A
{
    public delegate void ExitHandler(object sender, int a);
    public event ExitHandler Exit;
    ...
    private void a_FormClosed(object sender, FormClosedEventArgs e)
    {
        OnExit(100);
    }

    protected virtual void OnExit(int a)
    {
        // take a reference to the event (incase it changes)
        var handler = Exit;
        if (handler != null)
        {
            handler(this, a);
        }
    }

}

public class B
{
    private A _a;

    public B()
    {
        _a = new A();
        _a.Exit += (sender, value) => my_fun(value);
    }

    private void my_fun(int x)
    {
        if(x==100)
            do_smth;
        ...
    }
}