Invoking methods with optional parameters through reflection

Question

I've run into another problem using C# 4.0 with optional parameters.

How do I invoke a function (or rather a constructor, I have the ConstructorInfo object) for which I know it doesn't require any parameters?

Here is the code I use now:

type.GetParameterlessConstructor()     .Invoke(BindingFlags.OptionalParamBinding |              BindingFlags.InvokeMethod |              BindingFlags.CreateInstance,              null,              new object[0],              CultureInfo.InvariantCulture); 

(I've just tried with different BindingFlags).

GetParameterlessConstructor is a custom extension method I wrote for Type.

Answer 1

According to MSDN, to use the default parameter you should pass Type.Missing.

If your constructor has three optional arguments then instead of passing an empty object array you'd pass a three element object array where each element's value is Type.Missing, e.g.

type.GetParameterlessConstructor()     .Invoke(BindingFlags.OptionalParamBinding |              BindingFlags.InvokeMethod |              BindingFlags.CreateInstance,              null,              new object[] { Type.Missing, Type.Missing, Type.Missing },              CultureInfo.InvariantCulture); 

Answer 2

Optional parameters are denoted by an ordinary attribute and are handled by the compiler.
They have no effect (other than a metadata flag) on the IL, and are not directly supported by reflection (except for the IsOptional and DefaultValue properties).

If you want to use optional parameters with reflection, you'll need to manually pass their default values.