93. Delegates#

Just like verbs are as important as nouns, methods are as important as objects. It could be argued that the whole reason that Strategy pattern exists is to allow developers to store and pass around methods as if they were values.

Delegates, in C#, can be viewed as a more compact implementation of Strategy pattern with a built-in syntax. Inversely, strategy pattern can be seen as a way to implement delegates in object oriented languages.

Key point

Delegates in C# let developers treat methods as objects, enabling methods to be passed, stored, and dynamically invoked. All while retaining static type-safety.

Just like objects have types, functions and methods have types too. While the value 2 might have the type int, a function called Increment (which, say, increments a number by one) might have the type int to int.

// A simple local function to increment a number by 1.
int Increment (int x) => x + 1;

In mathematics we say that the method Add maps from int to int. In programming we call this a ‘type signature’ and in C# we call it a method signature. When reasoning about programming we often use arrow-syntax which would denote this method signature as int -> int. All we’ve done is remove put the input before the output and remove all identifiers.

Delegates allow us to create types whose values are methods. Delegate instances are references to methods which gives us a way to treat methods as first-class entities that we can pass around, much like we pass around objects.

Important

A delegate is a type whose values are methods.

While Strategy requires us to define interfaces or abstract classes and their implementations, delegates define a type that encapsulate a single method signature which can be instantiated and associated with any method that matches this signature.

Since a delegate is a type and not a value, a delegate has to be defined directly in a namespace, or as a member of a class. We cannot define a delegate inside a method, just like how we cannot define a new class inside another method.

To define a delegate we use the keyword delegate then state the return type of the delegate type, followed by the name of the delegate type, which is finally followed by a parameter list.

delegate string IntToString(int value);

The delegate that we’ve defined maps from int to string. Any method possible method whose type signature claims that it takes an int and returns a string can be considered a value of the type defined by this delegate. Since this is a type, we can declare variables of this type like we would any other type.

IntToString converter1;
IntToString converter2;

When assigning a value to a variable of our delegate type, the value must be a method that satisfies the signature of the delegate type. Let’s, for example, say that we have two local functions called PositiveOrNegative and ToHex that both take ints and return strings.

string PositiveOrNegative(int number)
    => number > 0 ? "Positive" : "Negative";

string ToHex(int number)
    => number.ToString("X");

Important

Functions or methods have types, just like values have types. Just like the value 2 might have the type int the function Increment (which, say, increments a number by one) might have the type int to int. In mathematics we say that the method Add maps from int to int. In programming we call this a ‘type signature’ and, when reasoning about it sometimes denote it as int -> int. The local functions in the example above both map from int to string which could be denoted as int -> string.

So far, these methods are completely unrelated to our previously defined delegate IntToString. These are regular old local functions that cannot be passed around like variables. Let’s create instances of our delegate (IntToString) by declaring variables to be of that type and then associate them with our local functions.

IntToString convert1 = PositiveOrNegative;
IntToString convert2 = ToHex;

Warning

When assigning a method to a variable of a delegate type, we don’t invoke the method. So, avoid using parentheses after the method name.

Important

Delegates maintain static type safety. If we attempt to assign a method to a variable declared as a delegate, and the method doesn’t match the delegate’s signature, we will get a compile-time type error.

Our delegate instances can now be invoked just like regular methods can be invoked.

Console.WriteLine(convert1(15));
Console.WriteLine(convert2(15));

Positive

Unlike regular methods however, we can now also pass them around like any old variable.

Just like with Strategy pattern we can pass algorithms around and choose which one to execute at run-time.