OK, we hear that structcannot have a default parameterless constructor which is fine (http://stackoverflow.com/questions/333829/why-cant-i-define-a-default-constructor-for-a-struct-in-net). But documentation says "Each value type has an implicit default constructor that initializes the default value of that type." from http://msdn.microsoft.com/en-us/library/s1ax56ch.aspx
What's the difference between implicit default constructor and parameterless default constructor right now?
The implicit default constructor is the parameterless constructor which is automatically created by the compiler. The reason you can't create a parameterless constructor is because the default one already exists. I don't know why they chose to do it this way, and why you're not allowed to override it.