Question

Given a class like this:

public abstract class SomeClass
  {
      private string _name;
      private int _someInt;
      private int _anotherInt;

      public SomeClass(string name, int someInt = 10, int anotherInt = 20)
      {
          _name = name;
          _someInt = someInt;
          _anotherInt = anotherInt;
      }
  }

Is it possible using reflection to get the optional parameter’s default values?

Answer 1

Lets take a basic program:

class Program
{
    static void Main(string[] args)
    {
        Foo();
    }
    public static void Foo(int i = 5)
    {
        Console.WriteLine("hi"   +i);
    }
}

And look at some IL Code.

For Foo:

.method public hidebysig static void  Foo([opt] int32 i) cil managed
{
  .param [1] = int32(0x00000005)
  // Code size       24 (0x18)
  .maxstack  8
  IL_0000:  nop
  IL_0001:  ldstr      "hi"
  IL_0006:  ldarg.0
  IL_0007:  box        [mscorlib]System.Int32
  IL_000c:  call       string [mscorlib]System.String::Concat(object,
                                                              object)
  IL_0011:  call       void [mscorlib]System.Console::WriteLine(string)
  IL_0016:  nop
  IL_0017:  ret
} // end of method Program::Foo

For Main:

.method private hidebysig static void  Main(string[] args) cil managed
{
  .entrypoint
  // Code size       9 (0x9)
  .maxstack  8
  IL_0000:  nop
  IL_0001:  ldc.i4.5
  IL_0002:  call       void ConsoleApplication3.Program::Foo(int32)
  IL_0007:  nop
  IL_0008:  ret
} // end of method Program::Main

Notice that main has 5 hardcoded as part of the call, and in Foo. The calling method is actually hardcoding the value that is optional! The value is at both the call-site and callee site.

You will be able to get at the optional value by using the form:

typeof(SomeClass).GetConstructor(new []{typeof(string),typeof(int),typeof(int)})
.GetParameters()[1].RawDefaultValue

On MSDN for DefaultValue (mentioned in the other answer):

This property is used only in the execution context. In the reflection-only context, use the RawDefaultValue property instead. MSDN

And finally a POC:

static void Main(string[] args)
{
   var optionalParameterInformation = typeof(SomeClass).GetConstructor(new[] { typeof(string), typeof(int), typeof(int) })
   .GetParameters().Select(p => new {p.Name,  OptionalValue = p.RawDefaultValue});

   foreach (var p in optionalParameterInformation)
      Console.WriteLine(p.Name+":"+p.OptionalValue);

   Console.ReadKey();
}

http://bartdesmet.net/blogs/bart/archive/2008/10/31/c-4-0-feature-focus-part-1-optional-parameters.aspx