Why can fixed size buffers only be of primitive types?

We have to interop with native code a lot, and in this case it is much faster to use unsafe structs that don't require marshaling. However, we cannot do this when the structs contain fixed size buffers of nonprimitive types. Why is it a requirement from the C# compiler that fixed size buffers are only of the primitive types? Why can a fixed size buffer not be made of a struct such as:

[StructLayout(LayoutKind.Sequential)]
struct SomeType
{
  int Number1;
  int Number2;
}

Solution

Fixed size buffers in C# are implemented with a CLI feature called "opaque classes". Section I.12.1.6.3 of Ecma-335 describes them:

Some languages provide multi-byte data structures whose contents are manipulated directly by address arithmetic and indirection operations. To support this feature, the CLI allows value types to be created with a specified size but no information about their data members. Instances of these “opaque classes” are handled in precisely the same way as instances of any other class, but the ldfld, stfld, ldflda, ldsfld, and stsfld instructions shall not be used to access their contents.

The "no information about their data members" and "ldfld/stfld shall not be used" are the rub. The 2nd rule puts the kibosh on structures, you need ldfld and stfld to access their members. The C# compiler cannot provide an alternative, the layout of a struct is a runtime implementation detail. Decimal and Nullable<> are out because they are structs as well. IntPtr is out because its size depends on the bitness of the process, making it difficult for the C# compiler to generate the address for the ldind/stind opcode used to access the buffer. Reference types references are out because the GC needs to be able to find them back and can't by the 1st rule. Enum types have a variable size that depend on their base type; sounds like a solvable problem, not entirely sure why they skipped it.

Which just leaves the ones mentioned by the C# language specification: sbyte, byte, short, ushort, int, uint, long, ulong, char, float, double or bool. Just the simple types with a well defined size.