C# out parameter performance

Question

Do out parameters in C# have any performance implications I should know about? (Like exceptions)

I mean, is it a good idea to have

Answer 1

There are no performance implications. out is basically the same as any old argument passing, from a technical point of view. While it might sound plausible that huge amounds of data are copied (e.g. for large structs), this is actually the same as for return values.

In fact, return values for all types > 32 bit are handled similar to out arguments on the machine level anyway.

Please note that the last statement doesn't suggest that returning a value == out parameter in .NET. Jon's benchmark shows that this is obviously (and regrettably) not the case. In fact, to make it identical, named return value optimization is employed in C++ compilers. Something similar could potentially be done in future versions of the JIT to improve performance of returning large structures (however, since large structures are quite rare in .NET, this might be an unnecessary optimization).

However, (and with my very limited knowledge of x86 assembly), returning objects from function calls generally entails allocating sufficient space at the call site, pushing the address on the stack and filling it by copying the return value into it. This is basically the same that out does, only omitting an unnecessary temporary copy of the value since the target memory location can be accessed directly.