Optional Parameter in C#

Introduction

This article explains the various ways to make method parameters optional. This is a very common interview question in C# Interview Questions.

There are following 4 ways to make method parameters optional.

Using parameter arrays
Using method overloading
Using parameter defaults
Using OptionalAttribute that is present in the System.Runtime.InteropServices namespace

Using parameter arrays

using System;

namespace OptionalParameter
{
    class Program
    {
        public static void Sum(int firstNumber, int secondNumber, params object[] restOfTheNumbers)
        {
            int result = firstNumber + secondNumber;
            foreach (int i in restOfTheNumbers)
            {
                result += i;
            }

            Console.WriteLine("Total = " + result.ToString());
        }

        public static void Main()
        {
            // Sum of two numbers
            Sum(10, 20);

            // Sum of more than two numbers
            Sum(10, 20, 30, 40, 50);
        }
    }
}

The Sum method allows us to add 2 or more numbers. The firstNumber and secondNumber parameters are mandatory, whereas the MoreNumbers parameter is optional.

Output

optonal param

Note. A parameter array must be the last parameter in a formal parameter list. The following method will not compile and gives a compile time error.

public static void Sum(int firstNumber, params object[] moreNumbers, int secondNumber)
{
    // Function implementation
}

Using method overloading

using System;

namespace OptionalParameter
{
    class Program
    {
        public static void Sum(int firstNumber, int secondNumber, int[] moreNumbers)
        {
            int result = firstNumber + secondNumber;
            if (moreNumbers != null)
            {
                foreach (int i in moreNumbers)
                {
                    result += i;
                }
            }

            Console.WriteLine("Sum = " + result);
        }

        public static void Sum(int firstNumber, int secondNumber)
        {
            int result = firstNumber + secondNumber;
            Console.WriteLine("Sum = " + result);
        }

        public static void Main()
        {
            // Sum of two Numbers
            Sum(10, 20);

            // Sum of more than two numbers
            Sum(10, 20, new int[] { 30, 40, 50 });
        }
    }
}

In the preceding example, I have two Sum methods that are overloaded or we can say there are 2 versions of the Sum method.

Output

method Overload

Parameters optional by specifying parameter defaults

using System;

namespace OptionalParameter
{
    class Program
    {
        public static void Sum(int firstNumber, int secondNumber, int[] moreNumbers = null)
        {
            int result = firstNumber + secondNumber;
            if (moreNumbers != null)
            {
                foreach (int i in moreNumbers)
                {
                    result += i;
                }
            }

            Console.WriteLine("Sum = " + result);
        }

        public static void Main()
        {
            // Sum of two Numbers
            Sum(10, 20);

            // Sum of more than two numbers
            Sum(10, 20, new int[] { 30, 40, 50 });
        }
    }
}

In the preceding example, I have set MoreNumbers=null by default and when I call Sum(10,20) then by default the value passed to MoreNumber=Null and when I call the same method like Sum(10,20, new int[]{30,40,50}) then it changes the default value.

Output

param dedfault

Example

using System;

namespace OptionalParameter
{
    class Program
    {
        public static void Test(int a, int b = 5, int c = 10)
        {
            Console.WriteLine("a = " + a);
            Console.WriteLine("b = " + b);
            Console.WriteLine("c = " + c);
        }

        public static void Main()
        {
            // a = 3, b = 5, c = 10
            Test(3);

            // a = 2, b = 3, c = 10
            Test(2, 3);

            // a = 5, b = 4, c = 6
            Test(5, 4, 6);
        }
    }
}

In the preceding example, the Test method contains the 3 parameters a, b, c but we have set b=5 and c=10 so here a parameter will be a required parameter and b and c are optional parameters.

test method

Note. Optional parameters must appear after all the required parameters, in other words the following method will not compile.

Assigning a value to a specific optional parameter

The following sample shows how to assign a value to a specific optional parameter

using System;

namespace OptionalParameter
{
    class Program
    {
        public static void Test(int a, int b = 5, int c = 10)
        {
            Console.WriteLine("a = " + a);
            Console.WriteLine("b = " + b);
            Console.WriteLine("c = " + c);
        }

        public static void Main()
        {
            
            Test(3);
            Console.WriteLine();
            Test(2, c: 3);
        }
    }
}

Output

example 2

Making method parameters optional using Optional Attribute

In this example, we will discuss OptionalAttribute in the System.Runtime.InteropServices namespace.

using System;
using System.Runtime.InteropServices;

namespace OptionalParameter
{
    class Program
    {
        public static void Sum(int firstNumber, int secondNumber, [Optional] int[] moreNumbers)
        {
            int result = firstNumber + secondNumber;
            if (moreNumbers != null)
            {
                foreach (int i in moreNumbers)
                {
                    result += i;
                }
            }
            Console.WriteLine("Sum = " + result);
        }

        public static void Main()
        {
            // Sum of two Numbers
            Sum(10, 20);

            // Sum of more than two numbers
            Sum(10, 20, new int[] { 30, 40, 50 });
        }
    }
}

In the preceding example, we have a method Sum where two parameters are required, firstNumber and secondNumber, but the last parameter MoreNumber I have marked as an [Optional] attribute.

Output

addtional attribute

Thank you. Happy Coding

7 Comments

Join the conversation! Your thoughts help the community grow.

Prabu Elavarasan7y ago
Is this Optional keyword works with REF & OUT properties?? Because in called method we cannot user ref int x=null right? Like said, is this rule applicable for Optional keyword as well?
Guest User12y ago
cool. simple & clear explanation!
- Sourabh Somani12y ago
  Sumit Jolly Sir: Thank you :)
Keyur Patel12y ago
Nice explanation.
- Sourabh Somani12y ago
  Thank you :)
Vithal Wadje12y ago
good one
- Sourabh Somani12y ago
  Thank you :)