C# – Debug vs. Release performance


I've encountered the following paragraph:

“Debug vs. Release setting in the IDE when you compile your code in Visual Studio makes almost no difference to performance… the generated code is almost the same. The C# compiler doesn’t really do any optimization. The C# compiler just spits out IL… and at the runtime it’s the JITer that does all the optimization. The JITer does have a Debug/Release mode and that makes a huge difference to performance. But that doesn’t key off whether you run the Debug or Release configuration of your project, that keys off whether a debugger is attached.”

The source is here and the podcast is here.

Can someone direct me to a Microsoft article that can actually prove this?

Googling "C# debug vs release performance" mostly returns results saying "Debug has a lot of performance hit", "release is optimized", and "don't deploy debug to production".

Best Solution

Partially true. In debug mode, the compiler emits debug symbols for all variables and compiles the code as is. In release mode, some optimizations are included:

  • unused variables do not get compiled at all
  • some loop variables are taken out of the loop by the compiler if they are proven to be invariants
  • code written under #debug directive is not included, etc.

The rest is up to the JIT.

Full list of optimizations here courtesy of Eric Lippert.