When maintaining software, you can’t always use the latest versions of your tools and languages.
Since C# has added a lot since version 1, it is handy to know what changed in which version.
Of course nothing beets the (paper and electronic!) editions of C# in Depth (covering C# 1, 2 and 3) and C# in Depth second edition (covering C# 2, 3 and 4).
Those books are thick (they need to, there is a truckload to cover!).
(The publisher (Manning) actually did a very good job on the e-book versions. Highly recommended, as it makes searching stuff so much easier).
The answers to the c# – List of new features in C#2.0, 3.0 and 4.0 Stack Overflow question contains a few concise lists of major changes in the language.
I really like the Bluffer’s Guide to C# 2, and the Bluffer’s Guide to C# 3 that Jon Skeet mentioned in his answer (and wish he wrote a Bluffer’s Guide to C# 4 as well), as they focus on some essential stuff in a very concise way.
If you want more detailed info, start with these stackoverflow questions:
Since stackoverflow wasn’t there when C# 2 was introduced in 2005, start with this nice C# 2 overview article on CSharpCorner.
Note: contrary to popular belief, there is no C# 3.5. There is .NET 3.5 (adding LINQ) and .NET 3.0 (adding WPF, WCF and WF) all based on the C# 3.0 and CLR 2 SP1 features.
(Yes, some expression tree features don’t work on CLR2 RTM, as that one has a few bugs, see Jared’s comment on this answer).
–jeroen
via: c# – List of new features in C#2.0, 3.0 and 4.0 – Stack Overflow.
The Wiert Corner - irregular stream of stuff 