The .NET framework is a software framework designed mainly for the Microsoft Windows operating system. It includes an implementation of the Base Class Library, Common Language Runtime (commonly referred to as CLI) and Dynamic Language Runtime. It supports many programming languages, including C#, VB.NET, F# and C++.
The .NET Framework is not specific to any one programming language. The C#, VB.NET, C++/CLI, and F# programming languages from Microsoft, as well as many other languages from other vendors, all use the same .NET Framework.
The .NET Framework includes a large library of functions as part of the Base Class Library (BCL), including those related to user interface design, data access, database connectivity, cryptography, development of web applications, mathematical algorithms, and network communications. This extensive library simplifies development and makes it easy to rapidly develop new applications.
As such, questions like "How do I do file I/O in C#?" should really be "How do I do file I/O with .NET?". Because such a question concerns the file I/O libraries provided by the .NET Framework, it should be tagged with both the .net tag, and with the c# tag, to indicate that you'd like the answer to be written in the C# programming language.