What is the params keyword in C#?

The params keyword

C# provides the params keyword to define a method parameter that can take a variable number of arguments.

Since there may be instances where a developer is not entirely sure of the number of arguments a method should have, the developer can use the params keyword to see if the caller can call our method with one or two or multiple arguments.

We can pass multiple arguments for the params parameter, and the compiler will change these arguments to a temporary arrayused to get the method parameters passed by the caller.

Therefore, the params parameter results in cleaner code, as we do not need to provide multiple overloads of a method.

With the params keyword for a method argument, the caller can pass an array of values or a comma-separated list of values in the function.

Some important notes

The params keyword marks the final argument in the method, so no other parameters can be added after using the params keyword in a method declaration.

//Invalid-params should be the last parameter
public int Sum(params int[] numbers, char input)

There can only be one params keyword argument in a method.

//Invalid- only one params is allowed
public int Sum(params int[] numbers, params int[] inputValues)

A compiler time error is shown if the declared type of params parameter is not a one-dimensional array.

Calling the params method

The caller can call a method with a params parameter, with:

An array of arguments of the specified type.
A comma-separated argument list of the type of the array elements.
We can skip passing the arguments and keep the argument list empty.

In the code sample below, we have created a function with the params parameter and called it with two arguments, five arguments, and an array of numbers. The function sums all the numbers passed and returns the total sum as the output.

Sum of  numbers is 9
Sum of numbers is 15
Sum of Array of nums  is 49

Code

using System;
namespace Hello
{
    class ParamsTest
    {
        static void Main(string[] args)
        {
            int twoSum = SumNumbers(4,5);
            Console.WriteLine("Sum of  numbers is {0}",twoSum);
            int fiveSum = SumNumbers(1, 2, 3, 4, 5);
            Console.WriteLine("Sum of numbers is {0}",fiveSum);
            int[] nums = { 2,5,3,4,5,6,7,8,9};
            int arraySum = SumNumbers(nums);
            Console.WriteLine("Sum of Array of nums  is {0}",arraySum);
        }
        public static int SumNumbers(params int[] numbers)
        {
            int totalSum = 0;            
            foreach (int number in numbers)
            {
                totalSum += number;
            }
            return totalSum;
        }
    }
}

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