File with a .CS extension contains source code of a program, stored in C# - C Sharp language, it is a text file. It can be opened and edited in any text editor, however, usually for this purpose a specialised programming editor with additional functions (syntax highlighting, suggestions or auto-filling) is used. CS files may contains complete simple programs are they may be elements of bigger, more complex applications or libraries.

C # language is an object-oriented programming language for the .NET environment, created by Microsoft on the basis of C and C ++. With Microsoft Visual Studio application, different programs can be built, including also in Visual C #. Initiating compiled applications is possible in the .NET Framework environment with the Common Language Runtime – CLR. CS files in a programming environment occur in a project along with other files constituting a complete program.