mjhjmtu

I used the file command on a c# source file, and linux thought it was a c++ file. What is the reason for this?

Take a look at the man page for the file command:

$ man file

...

file tests each argument in an attempt to classify it.
There are three sets of tests, performed in this order: filesystem
tests, magic tests, and language tests. The first test that succeeds
causes the file type to be printed.

It's the third test (language tests) that file performs that's categorizing this file as a C++ file.

Once file has determined the character set used in a text-type file,
it will attempt to determine in what language the file is written.
The language tests look for particular strings (cf. #include
) that can appear anywhere in the first few blocks of a file. For example, the keyword .br indicates that the file is most likely a
troff(1) input file, just as the keyword struct indicates a C program.
These tests are less reliable than the previous two groups, so they are performed last. The language test routines also test for some
miscellany (such as tar(1) archives).

C# files most closely match to C++ files and so file "guesses" that the .cs file is a C++ file.

Example

$ more blah.cs
// A Hello World! program in C#.
using System;
namespace HelloWorld

 class Hello
 
 static void Main()
 
 Console.WriteLine("Hello World!");

 // Keep the console window open in debug mode.
 Console.WriteLine("Press any key to exit.");
 Console.ReadKey();
 
 

Checking with file:

$ file blah.cs
blah.cs: ASCII C++ program text