C#

(language)/see sharp/ An object-oriented language devisedand promoted by Microsoft, intended to replace Java, whichit strongly resembles.

http://csharpindex.com/.

C#

(C sharp) An object-oriented programming language that is based on C++ with elements from Visual Basic and Java. Like Java, C# provides automatic garbage collection, whereas traditional C and C++ do not. C# was created by Microsoft and also standardized by the European Computer Manufacturers Association (ECMA). Microsoft designed C# as its flagship programming language for the .NET environment.

Why the Number Sign?
Because it is similar to the sharp music notation symbol, the number sign (#) is used. See .NET Framework, J#, F# and C.