.NET is Compile Time OR RunTime Environment?  

.Net's framework has CLS,CTS and CLR.CTS checks declaration of types at the time when u write code and CLS defines some rules and restrictions. And CLR comile everything at runtime with following benefits:

Vastly simplified development

Seamless integration of code written in various languages

Evidence-based security with code identity

Assembly-based deployment that eliminates DLL Hell

Side-by-side versioning of reusable components

Code reuse through implementation inheritance

Automatic object lifetime management

Self describing objects