What are Local functions in C# 7.0?

Local functions are private methods defined inside another member (method, constructor, or property). They are only accessible within their containing member and provide a clean way to break down complex logic into smaller, reusable pieces without exposing helper methods to the entire class.

Local functions were introduced in C# 7.0 and offer better performance than lambda expressions for recursive scenarios and provide compile-time checking for definite assignment.

Syntax

Following is the syntax for declaring a local function −

returnType LocalFunctionName(parameters) {
    // function body
}

Local functions can be declared in and called from −

• Methods, especially iterator methods and async methods

• Constructors

• Property accessors

• Event accessors

• Anonymous methods

• Lambda expressions

• Finalizers

• Other local functions

Using Local Functions for Simple Operations

Example

using System;

class Program {
    public static void Main() {
        void addTwoNumbers(int a, int b) {
            Console.WriteLine(a + b);
        }
        addTwoNumbers(1, 2);
    }
}

The output of the above code is −

3

Using Local Functions with Out Parameters

Example

using System;

class Program {
    public static void Main() {
        void addTwoNumbers(int a, int b, out int c) {
            c = a + b;
        }
        addTwoNumbers(1, 2, out int c);
        Console.WriteLine(c);
    }
}

The output of the above code is −

3

Local Functions Accessing Local Variables

Local functions can access variables from their containing method, creating closures −

Example

using System;

class Program {
    public static void Main() {
        int multiplier = 5;
        
        int MultiplyByFive(int number) {
            return number * multiplier;
        }
        
        Console.WriteLine("3 x 5 = " + MultiplyByFive(3));
        Console.WriteLine("7 x 5 = " + MultiplyByFive(7));
    }
}

The output of the above code is −

3 x 5 = 15
7 x 5 = 35

Local Functions vs Lambda Expressions

Conclusion

Local functions in C# 7.0 provide a clean way to create helper methods that are only needed within a specific scope. They offer better performance than lambda expressions for recursive operations and can access local variables from their containing method, making code more organized and maintainable.