The params keyword

C# course · Beginner · Lesson 79/116

Learn methods that take a variable number of arguments with params.

The params keyword

The params keyword lets a method take a variable number of arguments. The parameter must be of an array type and come last in the list. Declaration: static int Sum(params int[] nums).

At the call site, you can give any number of arguments: Sum(1, 2, 3), Sum(1, 2, 3, 4) or Sum(). The compiler packs the arguments into an array automatically.

params constraints

The params parameter must be the last in the list; it gathers all remaining arguments at the call. A method can have only one params parameter. Its type must be an array, such as int[] or string[].

Inside the body, the params parameter is used like a normal array: you can iterate with foreach (int n in nums) and read its length with nums.Length.

Exercise Sum with params

Define a static int Sum(params int[] nums) method; in the body iterate with foreach (int n in nums), sum all elements and return the result. In Main, call Sum(5, 10, 15) and print the result with Console.WriteLine.

Show example solution

Add foreach (int n in nums) total += n; and return total; to the Sum body, write Console.WriteLine(Sum(5, 10, 15)); in Main.
Example solution:
using System;

class Program
{
    static int Sum(params int[] nums)
    {
        // Sum the items in nums and return
        int total = 0;
        foreach (int n in nums) total += n;
        return total;
    }

    static void Main()
    {
        // Call Sum and print
        Console.WriteLine(Sum(5, 10, 15));
    }
}

Test yourself

Match each expression to its meaning

params int[]

Variable args
Sum(1, 2, 3)

Three args
Sum()

Zero args
params

Must be last

Declare params

static int Sum(____)
{
    int total = 0;
    foreach (int n in nums) total += n;
    return total;
}

Show answer

static int Sum(params int[] nums)
{
    int total = 0;
    foreach (int n in nums) total += n;
    return total;
}

What does Sum(1, 2, 3, 4) print?

static int Sum(params int[] nums)
{
    int total = 0;
    foreach (int n in nums) total += n;
    return total;
}
Console.WriteLine(Sum(1, 2, 3, 4));

The params keyword