https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-7

C# 7 features:
1.) out variables
2.) Tuples
3.) Discards
4.) Pattern Matching
5.) ref locals and returns
6.) Local Functions
7.) More expression-bodied members
8.) throw Expressions
9.) Generalized async return types
10.) Numeric literal syntax improvements
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https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-6

C# 6 features:
1.) Read-only Auto-properties:
2.) Auto-Property Initializers:
3.) Expression-bodied function members:
4.) using static:
5.) Null - conditional operators:
6.) String Interpolation:
7.) Exception filters:
8.) nameof Expressions:
9.) await in catch and finally blocks:
10.) index initializers:
11.) Extension methods for collection initializers:
12.) Improved overload resolution:
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https://www.go4expert.com/articles/features-c-sharp-50-t30461/

C# 5 features:
1.) Async and await
2.) Lambda expressions
3.) Caller Information
4.) Windows Runtime Support
5.) Compiler API
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C# 4 features:
1.) Dynamic programming.
2.) Named parameters.
3.) Optional parameters.
4.) Covariance and contravariance for generics and delegates.
5.) Late Binding
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C# 3 features:
1.) LINQ (language-integrated query)
2.) Object initializers.
3.) Collection initializers.
4.) Local variable type inference.
5.) Anonymous types.
6.) Lambda expressions.
7.) Expression trees.
8.) Automatic properties.
9.) Implicit Type (var)

Read more http://dotnetcrunch.com/list-of-c-new-features-by-version/
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C# 2 features:
1.) Various "bits and bobs" described below. Partial types. Aliases. Static classes. Property access modifiers.
2.) Nullable types and the null coalescing operator.
3.) Delegate changes.
4.) Implementing iterators with yield statements.
5.) Generics.
6.) Anonymous Methods
7.) Partial Class
8.) Covariance and Contravariance