Why is there no cyclic layout issue for classes in C#?

public struct Unit
{
    Unit u;
}

Causes:

Struct member 'Unit.u' of type 'Unit' causes a cycle in the struct layout.

But

public class Unit
{
    Unit u;
}

compiles. I understand the problem I suppose. An endless cycle will be formed when referencing a Unit object since it will have to initialize another member Unit and so on. But why does the compiler restrict the problem just for structs? Doesn't the issue persist for class too? Am I missing something?