Understanding C# Events
I am surprised how often C# developers are confused about delegates and events. They sometimes forget how all C# events are based on delegates and what the delegates offer for C# events. As well, some are confused about how to publish and subscribe to events and how to pass information when raising an event from a publisher to subscriber. Read more to learn about understanding C# events
In this understanding C# events tutorial, I will explain how delegates are the foundation for C# events and show the two primary delegates that Microsoft has given us for creating our own events. I will also show you how to subscribe to your own events and even pass data back to the event handlers.
Understanding C# Events:
What
Delegates Do
Every single event in .NET, whether Microsoft created it or if it was created by someone else, is based on a .NET delegate. Delegates are one of the five types of types included with .NET – class, structure, interface, enumeration, and delegate.
At its foundation, delegates do two things:
- When created, it points to a method (instance or static) in a container (class or structure). For events, it points to an event hander method.
- It defines exactly the kind of methods that it can point to, including the number and types of parameters and also the return type.
Understanding C# Events:
Definition of a
Simple Delegate
public delegate int dgPointer(int a, int b);public delegate int dgPointer(int a, int b);
class Program
{
static void Main()
{
Adder a = new Adder();
dgPointer pAdder = new dgPointer(a.Add);
int iAnswer = pAdder(4, 3);
Console.WriteLine("iAnswer = {0}", iAnswer);
// Returns “iAnswer = 7”
}
}
public class Adder
{
public int Add(int x, int y)
{ return x + y; }
}Let’s assume we want to raise an event in the Adder class if the sum of the two numbers in Add() is a multiple of five (5). We can define an event based on the delegate. This event will be used to raise a notification to run event handlers assigned to it.
NOTE: All C# events in .NET are based on delegates.
NOTE: Wherever you want to raise an event, you must also define the event.
NOTE:You must never raise (publish) an event unless at least one object is listening (subscribing) to the event. In other words, the event must not equal null.
NOTE: A Microsoft Best Practice: All events should be defined starting with the word “On”.
public class Adder
{
public delegate void dgEventRaiser();
public event dgEventRaiser OnMultipleOfFiveReached;
public int Add(int x, int y)
{
int iSum = x + y;
if ((iSum % 5 == 0) && (OnMultipleOfFiveReached != null))
{ OnMultipleOfFiveReached(); }
return iSum;
}
}class Program
{
static void Main()
{
Adder a = new Adder();
a.OnMultipleOfFiveReached += new Adder.dgEventRaiser(a_MultipleOfFiveReached);
int iAnswer = a.Add(4, 3);
Console.WriteLine("iAnswer = {0}", iAnswer);
iAnswer = a.Add(4, 6);
Console.WriteLine("iAnswer = {0}", iAnswer);
Console.ReadKey();
}
static void a_MultipleOfFiveReached()
{
Console.WriteLine("Multiple of five reached!");
}
}
This code creates a new instance of the dgEventRaiser delegate that points to the new method called a_MultipleOfFiveReached. The += operator is used to protect other methods that may have already subscribed to the event.
a.OnMultipleOfFiveReached += a_MultipleOfFiveReached;Here are the two built-in delegates:
public delegate void EventHandler(object sender, EventArgs e);
public delegate void EventHandler<TEventArgs>(object sender, TEventArgs e);The first delegate is used simply to raise a notification, an event signifying that something happened. The second delegate allows you to return one or more values to the event handler method. It requires you to create an instance of a class that derives from the EventArgs class.
To modify our code to use the first built-in delegate, we can delete our delegate and change our C# event to use the EventHandler delegate. When we raise the event, we must follow along with the delegate definition and pass in the required parameter values. Note how we pass our current instance of adder for the first parameter (sender) and since we are not passing back any event arguments, we use EventArgs.Empty for “e”.
We also had to change our event handler method to follow the pattern of the delegate with (object sender, EventArgs e).
class Program
{
static void Main()
{
Adder a = new Adder();
a.OnMultipleOfFiveReached += a_MultipleOfFiveReached;
int iAnswer = a.Add(4, 3);
Console.WriteLine("iAnswer = {0}", iAnswer);
iAnswer = a.Add(4, 6);
Console.WriteLine("iAnswer = {0}", iAnswer);
Console.ReadKey();
}
static void a_MultipleOfFiveReached(object sender, EventArgs e)
{
Console.WriteLine("Multiple of five reached!");
}
}
public class Adder
{
public event EventHandler OnMultipleOfFiveReached;
public int Add(int x, int y)
{
int iSum = x + y;
if ((iSum % 5 == 0) && (OnMultipleOfFiveReached != null))
{ OnMultipleOfFiveReached(this, EventArgs.Empty); }
return iSum;
}
}In order to use the other delegate and pass the grand total back to the event hander method, we first need to define a custom class called MultipleOfFiveEventArgs for passing back a custom value, such as Total. It must inherit from the EventArgs class.
Then we will need to define our event to use the other generic delegate which includes the custom EventArgs type, MultipleOfFiveEventArgs. We must also change how we raise the event. Finally, we change the event handler method to match the delegate.
Here is the complete code:
class Program
{
static void Main()
{
Adder a = new Adder();
a.OnMultipleOfFiveReached += a_MultipleOfFiveReached;
int iAnswer = a.Add(4, 3);
Console.WriteLine("iAnswer = {0}", iAnswer);
iAnswer = a.Add(4, 6);
Console.WriteLine("iAnswer = {0}", iAnswer);
Console.ReadKey();
}
static void a_MultipleOfFiveReached(object sender, MultipleOfFiveEventArgs e)
{
Console.WriteLine("Multiple of five reached: ", e.Total);
}
}
public class Adder
{
public event EventHandler<MultipleOfFiveEventArgs> OnMultipleOfFiveReached;
public int Add(int x, int y)
{
int iSum = x + y;
if ((iSum % 5 == 0) && (OnMultipleOfFiveReached != null))
{ OnMultipleOfFiveReached(this, new MultipleOfFiveEventArgs(iSum)); }
return iSum;
}
}
public class MultipleOfFiveEventArgs : EventArgs
{
public MultipleOfFiveEventArgs(int iTotal)
{ Total = iTotal; }
public int Total { get; set; }
}Conclusion
I hope this undestanding C# events tutorial has been helpful for you in learning delegates and events. Best wishes and happy programming!
GET TO KNOW US
Davin Mickelson
Black Slate Instructor & Consultant
Davin is a lead instructor at Black Slate and author of several blog posts on topics that include everything you would ever need to know about Microsoft software technologies.
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