If Int32 is just an alias for int, how can the Int32 class use an int?

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isn’t this illegal in C#? If “int” is only alias for “Int32” it should fail to compile with error CS0523. Is there some magic in the compiler?

Yes; the error is deliberately suppressed in the compiler. The cycle checker is skipped entirely if the type in question is a built-in type.

Normally this sort of thing is illegal:

struct S { S s; int i; }

In that case the size of S is undefined because whatever the size of S is, it must be equal to itself plus the size of an int. There is no such size.

struct S { S s; }

In that case we have no information from which to deduce the size of S.

struct Int32 { Int32 i; }

But in this case the compiler knows ahead of time that System.Int32 is four bytes because it is a very special type.

Incidentally, the details of how the C# compiler (and, for that matter, the CLR) determines when a set of struct types is cyclic is extremely interesting. I’ll try to write a blog article about that at some point.

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