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.
As explained in the answer you point to, using IL, you can define a parameterless constructor for a struct, but there are scenarios where it is not called.
IMHO the "implicit" constructor is just a logical one; implied by the memory being zeroed out when it is allocated.
Very old question, but to add more to the answer, the implicit constructor is compiled to a different instruction.
Given this C# code:
C c = new();
Console.WriteLine(c);
struct C(int A, int B) { public int a = A; public int b = B; }
the corresponding CIL code is this:
// ...
IL_0000: ldloca.s 0
IL_0002: initobj C // <---
IL_0008: ldloc.0
/// ...
While using the defined constructor uses an entirely different instruction that allocates the needed memory:
// ...
IL_0000: ldc.i4.1
IL_0001: ldc.i4.1
IL_0002: newobj instance void C::.ctor(int32, int32) // <---
// ...
It would seem like initobj does not actually allocate memory (don't quote this though.)
