95. Built-in delegates

When exploring delegates you might have noticed that it’s actually possible to assign any method to a variable if we use type inference.

// Local function that inverts a bool.
bool Negate (bool x) => !x;

// The function can be assigned to a variable *without* explicitly declaring a delegate.
var neg = Negate;

The below script needs to be able to find the current output cell; this is an easy method to get it.

How come this works? What is the type of variable neg? This works, because there’s a whole bunch of built-in delegate types in C# that the compiler can treat our methods as. The type of neg in the example above is Func<bool, bool>.

While we have the freedom to define custom delegates tailored to specific needs, the language also provides built-in generic delegate types for common scenarios. These types are intended to simplify method declarations, reduce code redundancy, and improve readability. The most prevalent built-in delegates are the Func types, the Action types, and Predicate<T>.

Key points

• C# offers built-in delegate types like Action, Func, and Predicate<T> for representing common delegates.

• The Action and Func families define generic delegates that return void or a value, respectively.

• The generic delegate Predicate<T> can be used for methods that take a single generic input and return a bool.

Action

The Action family of delegates contains delegates whose return type is void and can have up to 16 input parameters. When we have methods that perform an operation but do not return a value, Action is the go-to delegate family.

// Some example functions.
void Greet(string name) => Console.WriteLine("Hello, " + name);
void PrintCoordinates(int x, int y) => Console.WriteLine($"({x}, {y})");

// Using `Action` delegates.
Action<string> greet = Greet;
Action<int, int> printCoordinates = PrintCoordinates;
Action<string> printString = Console.WriteLine;
Action<int> printInt = Console.WriteLine;

// Invoking the delegates.
greet("Chris");
printCoordinates(2, 3);
printString("Hello");
printInt(10);

Hello, Chris

(2, 3)

Hello

10

Func

The Func family of delegates define generic delegates that can take up to 16 input parameters and always returns a value. Whenever we need a delegate for methods that compute and return a value, we can use a Func delegate.

// Some example functions.
int Add(int x, int y) => x + y;
int Invert(int x) => -x;

// Using `Func` delegates.
Func<int, int, int> add = Add;
Func<int, int> invert = Invert;
Func<string, int> convertToInt = Int32.Parse;

// Invoking the delegates.
Console.WriteLine(add(3, 2));
Console.WriteLine(invert(1));

5

-1

Predicate

The Predicate<T> delegate represents a method that takes a single input parameter and returns a bool value, making it ideal for conditions or filters.

// Some example functions.
bool IsEven(int x) => x % 2 == 0;
bool IsUpper(string str) => str.ToUpper() == str;

// Using `Predicate` delegates.
Predicate<int> isEven = IsEven;
Predicate<string> stringIsUpper = IsUpper;
Predicate<string> isNullOrEmpty = String.IsNullOrEmpty;

// Invoking the delegates.
Console.WriteLine(isEven(4));
Console.WriteLine(stringIsUpper("HELLO"));
Console.WriteLine(isNullOrEmpty("HELLO"));

True

True

False

Tip

For scenarios like filtering collections, Predicate<T> provides a type-safe and expressive way to pass conditions around. The Predicate<T> delegate is heavily used in .NET and you will e.g. find it in many operations that deal with collections such as the method FindAll available on List<T>.

Warning

While Predicate<T> and Func<T, bool> both are delegates that takes a parameter of type T and returns a bool, delegate instances of these types are not implicitly convertible to each other. This distinction exists because they represent different semantic meanings in the framework. Predicate<T> is specifically designed to represent a method that determines if an element of type T meets a condition. On the other hand, Func<T, bool> is a more general-purpose delegate for any method that takes a T and returns a bool. Always ensure you’re using the correct delegate type for the task at hand.

Why

Why should you use the built-in delegate types instead of your own custom types?

• Analyzability: By using well-recognized delegate types, you improve code clarity. Developers familiar with C# will immediately understand the purpose of a Func, Action, or Predicate delegate in your code.

• Reduced duplication: Instead of creating custom delegate types for each unique method signature, leveraging built-in delegates can often satisfy the same requirements.

• Interoperability: Many built-in C# libraries and frameworks make use of these delegates, making it essential to be familiar with them.

Why would you choose to write your own delegate instead of using a built-in type?

• Expressiveness: Custom delegate types can provide more meaningful names and clearer intentions. For instance, instead of a Func<Product, double>, a delegate called ProductPriceCalculation lets a developer know that the delegate’s purpose is to compute the price of a product. Similarly, instead of Func<List<Data>, List<Data>> a delegate called DataFilter clearly communicates that the purpose of the delegate is to filter data. By using your own types you can also specify more meaningful parameter names which greatly aids in using your code correctly. All this makes the code more self-documenting.

• Encapsulation: By creating your own delegate type, you encapsulate and centralize the definition. Instead of scattering a definition like Func<List<Data>, List<Data>> all over your codebase you’ve got it defined in one place which will likely improve maintainability.

• Fine-grained control: Custom delegates give you more control by allowing you to: define specific constraints, specify optional parameters, and use the ref, out, and the params keywords.

Conclusion

Built-in delegates in C# serve as versatile tools, providing type-safe and standardized ways to encapsulate method references. Whether you’re designing a custom API or just writing everyday code, understanding and utilizing Func, Action, and Predicate will significantly enhance the efficiency and clarity of your C# programming endeavors.

In upcoming chapters, we will delve deeper into how these built-in delegates come to life in advanced features like LINQ. Stay tuned!