Telling StructureMap to use another Constructor

If you call new MyClass(), then StructureMap is not involved at all. No amount of StructureMap configuration will change the behavior.

If you call ObjectFactory.GetInstance<MyClass>(), StructureMap will by default call the constructor with more parameters.

If you want StructureMap to use a different constructor, you can specify the constructor (via PHeiberg's answer):

x.SelectConstructor<IMyClass>(() => new MyClass(null));

Or you can just tell StructureMap explicitly how to create the instance using the overload of Use() that accepts a Func<>:

x.For<IMyClass>().Use(ctx => new MyClass(ctx.GetInstance<IMyService>()))