.NET

See C# Language Specification (link), the definitive source for C# syntax and usage. Direct download link (2.9 MB) - here.

Also C# Reference (link).

Concepts

• Algorithms and Data Structures in C# (link), Collections and Data Structures, .Net 4 (link)
• Boxing/Unboxing - see C# Programming Guide explanation.
• Collections (link)
• try/catch - link
• using Statement - link
• Generics: Introduction to C# Generics, MSDN Magazine (link)
• Iterators (link)
• Passing parameters in C# (link)

Some deprecated topics, might be still relevant for older versions of .NET:

• Code Access Security (link), in Practice (link); Is CAS Dead in .NET 4 (link).
• Security Changes in the .NET Framework 4 (link)