Introduction to Delegates in C#

Introduction to Delegates in C#

• Delegates in C#, created by using delegate keyword, are used as a type safe pointer, normally pointing towards C# Methods & Events.

• The signature of the Delegate & the function ( its pointing to ) must be same.
• As Delegate is a reference type Data type, so we can create its new instance.
• If you want to use any Method as a variable, then Delegates are literally the answer. ( We will have a look at it shortly )
• Delegates belong to builtin C# class named System.Delegate.
• Let's have a look at it's syntax:

• You can see in above figure that I have first declared the delegate, outside the class and I have used delegate keyword in its definition.
• Now in the Main function, I have created a new instance of this Delegate and in its constructor, I have pointed out the Method Name, to which I want to assign this delegate.
• After that I have invoked the Method using Delegate, instead of the Method's name itself.
• You need to make sure that signature of this delegate & the function must be same, when I use the word signature then that means return type and parameters.
• My Function & delegate, both have void return type, so if I change return type of any one of them, then compiler will generate an error and that's why we call it type safe.
• Similarly, the number & type of Function Parameters must have to be same for both Delegate & its pointed Method.
• Let's have a look at MultiCast Delegate in C#:

MultiCast Delegate in C#

• We can assign / point a single delegate to multiple Methods / Functions, such delegates are called MultiCast Delegates in C#.
• In the below figure, I have assigned a single delegate to 3 methods and when I invoked the delegate then it has executed all these 3 methods.

• In the above figure, I have used two ways to chain delegates together, in the first way I have used plus sign ( + ), while in second one I have used append sign ( += ).
• You must be wondering why to use these function pointers when we can directly invoke the Method using its own name.
• So, in order to understand the importance of delegates, let's design a simple example:

Example of Delegate in C#

• Let's create simple code, where we create a new class named StudentsData and it will have 3 C# Properties and 1 C# Method.
• You can see this code in below figure and I have placed an if loop in my method which has a Boolean expression in it i.e. score < 40.

• This condition Score < 40 is hard coded in my method, which makes my code quite rigid, whereas its always good practice to add flexibility in your code so that you could reuse it.
• So, now let's create a new method and add a delegate instead of this Boolean expression and as its a Boolean expression, so my delegate also has to be of Boolean return type.

• You can see in above figure that I have created a delegate named StudentsDelegate, which is of Boolean return type.
• After that, I have created a method in Main Function named CheckStatus, which is also of Boolean Type and taking Score as a parameter.
• If you look closely, the signature of this delegate & method CheckStatus are same.
• I have used the delegate to point to this method CheckStatus and have placed our failed condition in it.
• Now this function will return either TRUE or FALSE i.e. it's kind of a Boolean expression.
• Finally, I have added a delegate as a second parameter in FailedStudents Function, and then used this parameter as a condition in IF Loop.
• So, now my class is completely flexible, if I wanna change the condition, then I don't need to change the class, I can change score condition in my Main class.
• Now, you must be thinking, we have done so much coding just to shift a condition from one class to another, we have done it for a purpose.
• So, let's have a look at the below image, where I have used Lambda Expression in C#:

• In C#, we can use lambda expressions, which we will study in coming lectures, but for now you can see in above figure that I have removed the delegate instantiation and have also removed the pointed method.
• Instead I have just placed a Lambda expression and visual studio has created the delegate & method in the background, which we don't need to worry about.
• We just need to understand this lambda expression where we have placed our condition i.e. Score < 40.
• Delegates are the basis of this lambda expression, which is quite handy, it has not only made the class flexible but have also reduced the code to just single line.