Exploring internal implementation of C# 4.0 Default and Optional arguments

I wanted to explore how C#  compiler has implemented the optional and the default parameter. So here is the sample code that I wrote to verify.

using System;
namespace OptionalTest {
 internal class Program {
 private static void Main(string[] args) {
 var p = new Program();
 Test();
 }
 private static void Test(int x = 10) {
 Console.WriteLine(x);
 }
 }
}

As you can see the default parameter value for x is 10 and it is an optional parameter. After compiling the source code the next step was to disassemble with Reflector.

private static void Test([Optional, DefaultParameterValue(10)] int x){
Console.WriteLine(x);
}

Now it makes sense, looks like the C# compiler emits a custom attribute for System.Runtime.InteropServices.OptionalAttribute which marks the method to have an optional argument and the DefaultParameterValue stores the default value for the parameter. The next step was to disassemble the caller of this function.

And here is the disassembled Main method using reflector

private static void Main(string[] args)
{
Program p = new Program();
Test(10);
}

Notice we didn’t include 10 as an argument to method Test. So the compiler notices that the calling code does not have value for the argument and copies the value from the DefaultParameterValue and sticks it in.