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Just came across this:

Method Inlining

There is a cost associated with method calls; arguments need to be pushed on the stack or stored in registers, the method prolog and epilog need to be executed and so on. The cost of these calls can be avoided for certain methods by simply moving the method body of the method being called into the body of the caller. This is called Method In-lining. The JIT uses a number of heuristics to decide whether a method should be in-lined. The following is a list of the more significant of those (note that this is not exhaustive):

• Methods that are greater than 32 bytes of IL will not be inlined.
• Virtual functions are not inlined.
• Methods that have complex flow control will not be in-lined. Complex flow control is any flow control other than if/then/else; in this case, switch or while.
• Methods that contain exception-handling blocks are not inlined, though methods that throw exceptions are still candidates for inlining.
• If any of the method's formal arguments are structs, the method will not be inlined.

I would carefully consider explicitly coding for these heuristics because they might change in future versions of the JIT. Don't compromise the correctness of the method to attempt to guarantee that it will be inlined. It is interesting to note that the inline and __inline keywords in C++ do not guarantee that the compiler will inline a method (though __forceinline does).

Property get and set methods are generally good candidates for inlining, since all they do is typically initialize private data members.

HINT   Don't compromise the correctness of a method in an attempt to guarantee inlining.

From Writing High Performance Managed Applications