Where are operators defined in C# ?

Well, the current compiler is a 'black box' written in (unmanaged) Visual C++ and so we don't really know much about its innards at all.

However, the C# compiler has recently been rewritten in C# itself (as part of the Roslyn project) and open sourced. You can therefore browse the source code if you like:

http://roslyn.codeplex.com/

What I am looking is how C# compiler knows if '+' operator exist but '+++++' does not? Does any namespace is having operator information?

All the operators used by C# are defined in the C# Specification the current version of which (5.0) can be downloaded from here:

http://www.c-sharpcorner.com/Ebooks/Free/83/

The rules for using these operators are built into the compiler as is the grammar for the language as a whole.