Why is TypedReference behind the scenes? It’s so fast and safe… almost magical!

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Short answer: portability.

While __arglist, __makeref, and __refvalue are language extensions and are undocumented in the C# Language Specification, the constructs used to implement them under the hood (vararg calling convention, TypedReference type, arglist, refanytype, mkanyref, and refanyval instructions) are perfectly documented in the CLI Specification (ECMA-335) in the Vararg library.

Being defined in the Vararg Library makes it quite clear that they are primarily meant to support variable-length argument lists and not much else. Variable-argument lists have little use in platforms that don’t need to interface with external C code that uses varargs. For this reason, the Varargs library is not part of any CLI profile. Legitimate CLI implementations may choose not to support Varargs library as it’s not included in the CLI Kernel profile:

4.1.6 Vararg

The vararg feature set supports variable-length argument lists and runtime-typed pointers.

If omitted: Any attempt to reference a method with the vararg calling convention or the signature encodings associated with vararg methods (see Partition II) shall throw the System.NotImplementedException exception. Methods using the CIL instructions arglist, refanytype, mkrefany, and refanyval shall throw the System.NotImplementedException exception. The precise timing of the exception is not specified. The type System.TypedReference need not be defined.

Update (reply to GetValueDirect comment):

FieldInfo.GetValueDirect are FieldInfo.SetValueDirect are not part of Base Class Library. Note that there’s a difference between .NET Framework Class Library and Base Class Library. BCL is the only thing required for a conforming implementation of the CLI/C# and is documented in ECMA TR/84. (In fact, FieldInfo itself is part of the Reflection library and that’s not included in CLI Kernel profile either).

As soon as you use a method outside BCL, you are giving up a bit of portability (and this is becoming increasingly important with the advent of non-.NET CLI implementations like Silverlight and MonoTouch). Even if an implementation wanted to increase compatiblility with the Microsoft .NET Framework Class Library, it could simply provide GetValueDirect and SetValueDirect taking a TypedReference without making the TypedReference specially handled by the runtime (basically, making them equivalent to their object counterparts without the performance benefit).

Had they documented it in C#, it would have had at least a couple implications:

  1. Like any feature, it may become a roadblock to new features, especially since this one doesn’t really fit in the design of C# and requires weird syntax extensions and special handing of a type by the runtime.
  2. All implementations of C# have to somehow implement this feature and it’s not necessarily trivial/possible for C# implementations that don’t run on top of a CLI at all or run on top of a CLI without Varargs.

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