This calling convention is not supported in this version of the .NET Framework.
Assembly: mscorlib (in mscorlib.dll)
The __fastcall calling convention specifies that arguments to functions be passed in registers, when possible.
The classes in System.Runtime.CompilerServices are for compiler writers' use only.
Compilers emit custom modifiers within metadata to change the way that the just-in-time (JIT) compiler handles values when the default behavior is not appropriate. When the JIT compiler encounters a custom modifier, it handles the value in the way that the modifier specifies. Compilers can apply custom modifiers to methods, parameters, and return values. The JIT compiler must respond to required modifiers but can ignore optional modifiers. A C++ compiler could emit a custom modifier to describe how a byte should be treated in cases where the JIT compiler treats bytes in manner that is not compatible with C++ by default.
You can emit custom modifiers into metadata using one of the following techniques:
Generating a Microsoft intermediate language (MSIL) instruction file that contains calls to modopt and modreq, and assembling the file with the MSIL Assembler (Ilasm.exe).
Using the unmanaged reflection API.
Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98
The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.