30. Methods#

Programming is all about breaking down complex tasks into smaller, manageable parts. Divide and conquer. One way we accomplish this is through the use of methods. Methods is a key building block in writing maintainable code. They allow us to write code once and reuse it in multiple places, making our programs less redundant.

In C#, the words ‘method’ and ‘function’ are used somewhat interchangeably. To simplify, we can understand both as a named block of code that might take some parameters, performs some task, and possibly returns a value.

Think of a method as a small machine in a factory: you feed it some raw materials (the inputs, known as ‘parameters’ or ‘arguments’), it performs a specific task (known as the ‘body’ of the method) using these materials, and it spits out a finished product (the output, or ‘return value’).

To introduce methods, we’re going to use a feature of C# known as ‘local functions’. All programs we’ve written so far has been written directly in the file Program.cs. Assuming that you haven’t disabled top-level statements (as mentioned in the chapter called Hello world) you should be able to write and use local functions directly in Program.cs. A method in C# looks like this:

void SayHello()
{
    Console.WriteLine("Hello, World!");
}

In this example, SayHello is a method. When we call this method in our code, it performs its task: it writes "Hello, World!" to the console.

Having defined this method, we can now ‘call’ it. Calling a method is also known as ‘invoking’ or ‘running’ it.

SayHello();

Hello, World!

Parameters#

Methods can take parameters, which are like variables that get passed into the method. Parameters allow us to feed different inputs into our methods.

void Greet(string name)
{
    Console.WriteLine($"Hello, {name}!");
}

To call this method we must now also supply an argument that will serve as the value for the parameter name.

Greet("Chris");
Greet("World");

Hello, Chris!

Hello, World!

Since we specified that the name parameter must be of type string we cannot call this method and pass an argument of a different type.

Greet(true);

(1,7): error CS1503: Argument 1: cannot convert from 'bool' to 'string'

The error states that the bool that we passed cannot be implicitly converted into a string which is the type that was expected.

A method can take any number of parameters, separated by commas (,). This part of the method declaration is known as the ‘parameter list’.

In the following code example we are defining a method named Repeat.

void RepeatPrint(string input, int times)
{
    string output = "";
    for (int i = 1; i <= times; i++)
        output += input;
    Console.WriteLine(output);
}

If we call the method with the values Hello and 3 as arguments, the method will print HelloHelloHello to the console.

RepeatPrint("Hello", 3);

HelloHelloHello

Return values#

A method can also return a value to the part of the code that called it. This is specified using the return keyword.

Each method also specifies a return type. This is the type of the value that the method will return to the part of the code that called it. If a method doesn’t return a value, we use the keyword void.

In the code examples above the return type of the methods was void. There was no need to return anything since the methods themselves printed to screen.

Let’s now rewrite them so that they return values instead of printing on their own.

string MakeGreeting(string name)
{
    return $"Hello, {name}!";
}

Here, MakeGreeting is a method that takes one parameter: name, which is a string. It returns a string that is a greeting for the given name. Let’s also rewrite the RepeatPrint method to make it only repeat and not both repeat and print.

string Repeat(string input, int times)
{
    string output = "";
    for (int i = 1; i <= times; i++)
        output += input;
    return output;
}

Now that we’re using the return keyword our methods are more reusable than before. We can now use method composition to solve more complex problems without writing additional code.

MakeGreeting(Repeat("Chris", 2));

Note

A ‘local method’ in C# is a method that’s declared inside another method. In the examples in this chapter we’ve defined all methods in the Main method of the program. We will explain this in further detail in future chapters.