C# is a high level langauge that runs on the CLR (Common Language Runtime) Engine.

When you compile a C#, even though an exe file is created, it isnt 100% machine code, it still requires to be interpreted into machine code.

This is where the .NET or CLR runtime comes in. In order to run a C# program you require to have a version of Microsoft .NET installed. The latest versions of windows generally come with .NET already installed. If you try to execute a .NET program without .NET installed, it will tell you it requires the .NET package be downloaded.

All programs written on my site, will compile on .NET version 3.5.

Where to Get .NET & Versions

.NET 3.5 SP1

.NET 2

Microsoft .NET (All Versions)