C# doesn't use #include
All of the classes in all of the files are automatically available everywhere. If a class is created inside the same project namespace then it can be used everywhere. If it's in a different namespace you 'include' it with the "using" keyword.
There is no replacement for a C++ #include statement in C#. C# is an object-oriented language where code is organised into classes. You can use code from one class in another class depending on its visibility, and you can split the code from a single class across multiple source files using partial classes.