delegate example in C#

I've made a simple example to show how to create a delegate in C#. We use a delegate as callback function or an implementation of an observable pattern. When any event happens in our system, we what to have where we can write code to handle that event. This is what we need a delegate.

code example in C# and requires NUnit to run the test

using System;
using NUnit.Framework;

namespace SciApp.Utility.Test
{
    public delegate void MyCustomEventHandler(object sender, int countNumber, EventArgs eventArgs);
    public class CounterEventArgs : EventArgs
    {
        public int Number { get; set; }
    }

    public class Counter
    {
        private int number;
        public event MyCustomEventHandler ThresholdReached;

        public int Number
        {
            get
            {
                return number;
            }

            set
            {
                number = value;
                if (ThresholdReached != null)//always check null
                {
                    //set current value to number argument or pass as property of EventAgrs
                    var eventArgs = new CounterEventArgs() {Number = number};
                    ThresholdReached(this, number, eventArgs);
                }

            }
        }

    }

    [TestFixture]
    public class DelegateTest
    {
        private int currentCounterNumberValue;

        public void SetUp()
        {
            currentCounterNumberValue = 0;
        }

        [Test]
        public void ThresholdReached_ValidEventHanlder_CorrectNumberSentToTheHandler()
        {
            var counter = new Counter();
            counter.ThresholdReached += Counter_ThresholdReached;
            counter.Number += 2;
            Assert.AreEqual(2,currentCounterNumberValue);
        }

        private void Counter_ThresholdReached(object sender,int countNumber,EventArgs eventArgs)
        {
            //assert from parameter or event args object, you can make it as strongly type as well

            //currentCounterNumberValue = countNumber;
            currentCounterNumberValue=   ((CounterEventArgs)eventArgs).Number;
        }
    }
}