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    <p>Fixed size buffers in C# are implemented with a CLI feature called "opaque classes". Section I.12.1.6.3 of <a href="http://www.ecma-international.org/publications/standards/Ecma-335.htm">Ecma-335</a> describes them:</p> <blockquote> <p>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.</p> </blockquote> <p>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. Also the reason that you cannot use the <em>sizeof</em> operator on a struct. Decimal and Nullable&lt;> 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.</p> <p>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.</p>
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    1. POWhy can fixed size buffers only be of primitive types?
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    1. POWhy can fixed size buffers only be of primitive types?
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    1. COSince when is it true that "`IntPtr` is out"? `struct S { IntPtr p1, p2, p3; }` is completely unmanaged, and you can take `sizeof(S)` perfectly fine. Similarly with `struct P { void* p1, p2, p3; }` and `sizeof(P)`, etc... there's no reason why these should be prohibited (and they aren't). Your answer makes no sense to me.
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    2. COYou appear to be confusing the size problem with the can't-use-ldfld/stfld problem. They didn't write the code to generate the ldind/stind address from an expression that uses sizeof. Eric Lippert's answer to why-wasn't-that-done is always the same.
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    3. COThough I'm no C# guru, usually specs like that at the VM level are because of atomicity related to thread safety. The VM can atomically operate on primitives, but it cannot on any type of structure. It's the only way of keeping thread-safety guarantees in place.
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