As title, many thank !!
Unity does not use the .NET framework but instead a modified version of Mono which is comparable to .NET framework 3.5. Unity scripts are executed on the .NET 2.0 runtime.
Some developers may be interested to note that when using the Microsoft C# compiler with Visual Studio (or Express) you can actually use C# 4.0 syntax with the .NET 3.5 framework for the .NET 2.0 runtime which works fantastically with Unity 3 and 4.
Warning: Some features from the .NET 3.5 framework are not implemented in Mono and so you should be careful not to use functionality that is a little too exotic for Mono