The original name of the programming language that became C# was COOL, short for “C-like Object-Oriented Language.” Microsoft used the name during the early development of its .NET platform in the late 1990s.
In January 1999, Anders Hejlsberg formed a Microsoft team to create the new language. The project was intended to provide a modern, type-safe, object-oriented language for .NET, drawing on ideas from C++, Java, Eiffel, and earlier languages Hejlsberg had helped design, including Turbo Pascal and Delphi.
Microsoft eventually changed the name to C# before publicly presenting the .NET project in July 2000. COOL was not retained partly because of trademark concerns. The final name also signaled a relationship to C and C++: in music, a sharp raises a note by a semitone, while the symbol resembles four plus signs arranged together. Because keyboards and typefaces generally lack the musical sharp symbol, programmers write the language as C# using the number sign.