.NET is a platform independent but not fully platform independent. why because, the code written in any of the .NET Languages are compiled into MSIL code ( microsoft intermediate language code) . The MSIL code is platform independent. Hence to Run a .NET application on any machine we need to install .NET Framework. The .NET framework contains CLR(Common Language Runtime) , and CLR contains a JIT (Just-In-Time) Compiler. And it converts the MSIL code into machine code. And .NET Framework is released only for Windows Operating Systems.
But there is a third party framework “MONO” to run .NET applications on LINUX based Systems. Hence .NET is not a fully Platfrom Dependent.