events - Super-simple example of C# observer/observable with delegates

Question

I recently started digging into C# but I can't by my life figure out how delegates work when implementing the observer/observable pattern in the language.

Could someone give me a super-simple example of how it is done? I have googled this, but all of the examples I found were either too problem-specific or too "bloated".

Answer 1

The observer pattern is usually implemented with events.

Here's an example:

using System;

class Observable
{
    public event EventHandler SomethingHappened;

    public void DoSomething() =>
        SomethingHappened?.Invoke(this, EventArgs.Empty);
}

class Observer
{
    public void HandleEvent(object sender, EventArgs args)
    {
        Console.WriteLine("Something happened to " + sender);
    }
}

class Test
{
    static void Main()
    {
        Observable observable = new Observable();
        Observer observer = new Observer();
        observable.SomethingHappened += observer.HandleEvent;

        observable.DoSomething();
    }
}

See the linked article for a lot more detail.

Note that the above example uses C# 6 null-conditional operator to implement DoSomething safely to handle cases where SomethingHappened has not been subscribed to, and is therefore null. If you're using an older version of C#, you'd need code like this:

public void DoSomething()
{
    var handler = SomethingHappened;
    if (handler != null)
    {
        handler(this, EventArgs.Empty);
    }
}