Event Pattern in .NET

September 24, 2020   • c-sharp

Here is a simple example that illustrates how to use the event pattern in .NET.

First, define the structure of the data that you want to send when the event happens.

public class ThresholdReachedEventArgs : EventArgs
    public int Threshold { get; set; }
    public DateTime TimeReached { get; set; }

Then, define the Publisher, i.e. the class that wants to raise an event

public class Counter
    // 1. Define an event
    public event EventHandler<ThresholdReachedEventArgs> ThresholdReached;

    // 2. Do something that raises the event, and pass the EventArgs custom data 
    public void DoSomething()
        ThresholdReachedEventArgs e = new ThresholdReachedEventArgs
            Threshold = 10,
            TimeReached = DateTime.Now
        // 3. Raise the actual event
        ThresholdReached?.Invoke(this, e);

Finally, create one or many subscribers. Classes that want to listen to an event raised by the Publisher.

class Program
    static void Main(string[] args)
        // 1. Create an instance of the publisher, which you want to listen to
        Counter counter = new Counter();

        // 2. Attach an event handler on the publisher
        counter.ThresholdReached += OnThresholdReached;
        // 3. Do something that will raise the event. Now you are ready to listen to the event. 

    // 4. Handle the event which is raised by publisher
    static void OnThresholdReached(object sender, ThresholdReachedEventArgs e)
        Console.WriteLine($"Reached Threshold {e.Threshold} at {e.TimeReached.ToString()}");