Optional Parameters Do Not Create Default Constructors

At least not from the reflection API's point of view.

For example, given the following class ...

public class Wig
{
    public Wig(string name = "Angry")
    {
        // ...
    }
}

... you can successfully instantiate the class as if it had a default, parameterless constructor:

var wig = new Wig();

However, trying to do this via Activator will fail:

var wig = Activator.CreateInstance<Wig>();
// MissingMethodException: 
// No parameterless constructor defined for this object.

Which makes sense once you realize optional parameters are compiler sugar, and the following code will clearly show a single constructor requiring a single argument.

var ctors = typeof(Wig).GetConstructors();
foreach (var ctor in ctors)
{
    Console.WriteLine(ctor.Name);
    foreach(var param in ctor.GetParameters())
    {
        Console.WriteLine("\t{0} {1}", 
            param.Name, 
            param.ParameterType.Name );
    }                    
}