How to get default constructor when parameters are optional

The problem is that the C# compiler produces this:

public class B
{
    // Methods
    public B([Optional, DefaultParameterValue(0)] int i, [Optional, DefaultParameterValue("")] string str)
    {
    }
}

Something like below should work:

public static class TypeHelper {
    public static ConstructorInfo GetDefaultConstructor<TType>() {
        var type = typeof(TType);
        return type.GetDefaultConstructor();
    }

    public static ConstructorInfo GetDefaultConstructor(this Type type) {
        if(type == null) throw new ArgumentNullException("type");
        var constructor = type.GetConstructor(Type.EmptyTypes);
        if(constructor == null) {
            var ctors = 
                from ctor in type.GetConstructors()
                let prms = ctor.GetParameters()
                where prms.All(p=>p.IsOptional)
                orderby prms.Length
                select ctor;                        
            constructor = ctors.FirstOrDefault();
        }
        return constructor;
    }
}

The problem is that optional parameters are nothing more than a compile time concept. You'll need to specify the constructor completely.

var ci = typeof(B).GetConstructor(new [] { typeof(int), typeof(string) });

You can write a help function that will invoke the constructor with the default values though. My example is not as robust as it should be but it should get you started.

static Func<T> CreateDefaultConstructor<T>(ConstructorInfo ci)
{
    var l = new List<object>();
    foreach (var p in ci.GetParameters())
    {
        if (p.IsOptional)
        {
            l.Add(p.RawDefaultValue);
        }
    }
    return () => (T)ci.Invoke(l.ToArray());
}

Say I have the following class:

public class SomeClass
{
    public SomeClass()
    {

    }

    public SomeClass(int x)
    {
    }

    public SomeClass(int x = 0, int y = 0)
    {

    }
}

Basically, you're asking for a query that will find the constructors that match constructor 1 and 3 above? If so, use this:

var constuctors = typeof(SomeClass).GetConstructors()
            .Where(x => x.GetParameters().Count() == 0 
                    ||  x.GetParameters().Count(param => param.GetCustomAttributes(typeof(OptionalAttribute), false).Count() > 0) == x.GetParameters().Count());    

Incredibly nasty query, but it gets the job done returning only 1 and 3 above.