Question

In the following console application example, the event is defined like this:

public delegate void PurchaseHandler(object obj, PurchaseArgs args);
public event PurchaseHandler OnPurchaseMade;

It seems to me after reading around that this might be a bit “C#2“.

Is there a more abbreviated way to express this with C#3 and C#4?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace TestEvents288202
{
    class Program
    {
        static void Main(string[] args)
        {
            Product product1 = Product.LoadProduct(222);
            EmailManager.NotifyAdministrator(product1);
            product1.OnPurchaseMade += new Product.PurchaseHandler(NotifyUser);
            product1.Purchase();

            Product product2 = Product.LoadProduct(333);
            EmailManager.NotifyAdministrator(product2);
            product2.OnPurchaseMade += new Product.PurchaseHandler(NotifyUser);
            product2.Purchase();

            Console.ReadLine();
        }

        static void NotifyUser(object sender, PurchaseArgs e)
        {
            ((Product)sender).Log();
            Console.WriteLine(e.Message);
        }
    }

    public static class EmailManager
    {
        public static void NotifyAdministrator(Product product)
        {
            product.OnPurchaseMade += new Product.PurchaseHandler(SendEmail);
        }

        public static void SendEmail(object sender, PurchaseArgs e)
        {
            Product product = sender as Product;
            Console.WriteLine("Just sent e-mail to administrator notifying of purchase of article {0}", product.ProductNumber);
        }
    }

    public class PurchaseArgs : EventArgs
    {
        public string Message { get; set; }

        public PurchaseArgs(string message)
        {
            Message = message;
        }
    }

    public class Product
    {
        public int ProductNumber { get; set; }
        public string Name { get; set; }
        public string Description { get; set; }

        public delegate void PurchaseHandler(object obj, PurchaseArgs args);
        public event PurchaseHandler OnPurchaseMade;

        public static Product LoadProduct(int productNumber)
        {
            List<Product> products = new List<Product>();
            products.Add(new Product { ProductNumber = 111, Name = "Intel CPU", Description = "Newest model, very fast." });
            products.Add(new Product { ProductNumber = 222, Name = "Philips Monitor", Description = "22-inch, very nice." });
            products.Add(new Product { ProductNumber = 333, Name = "Sony Camera", Description = "10 Megapixels, sharp pictures." });

            return products.Where(p => p.ProductNumber == productNumber).SingleOrDefault();
        }

        public void Purchase()
        {
            PurchaseArgs purchaseArgs = new PurchaseArgs(String.Format("The product \"{0}\" was just purchased.", this.Name));
            OnPurchaseMade(this, purchaseArgs);
        }

        public void Log()
        {
            Console.WriteLine("Log: #{0} purchased.", this.ProductNumber);
        }
    }
}

Answer 1

Always define events like this, don’t use custom delegates:

event EventHandler<EventArgsClassType> MyEventHandler;

or, if they don’t take arguments:

event EventHandler MyEventHandler;

By being based on the System.EventHandler class, this ensures a uniform signature for all events, following the .NET guidelines.

If your event takes further arguments, its EventArgsClassType must inherit from System.EventArgs.

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Furthermore, when instantiating an event handler, you don’t have to use this explicit form:

product1.OnPurchaseMade += new Product.PurchaseHandler(NotifyUser);

since method groups can be implicitly converted to matching delegates. As a consequence, the following code works just as well:

product1.OnPurchaseMade += NotifyUser;