C# 1	Classes Structs Interfaces Events Properties Delegates Expressions Statements Attributes
C# 2	Generics Nullable value types Method group conversions Anonymous methods Delegate covariance and contravariance Iterators (yield return) Partial types Static classes Separate getter/setter access on properties Namespace alias qualifier :: syntax The global **namespace alias Extern aliases Fixed-size buffers InternalsVisibleToAttribute support
C# 3	Partial methods Automatically implemented properties Implicitly typed local variables (var) Implicitly typed arrays (new[]) Object initializers Collection initializers Anonymous types Lambda expressions (delegates) Lambda expressions (expression trees) Extension methods Query expressions
C# 4	Dynamic typing Optional parameters Named arguments Linked primary interop assemblies Special rules for optional parameters in COM Access to named indexers (COM only) Generic variance for interfaces and delegates Implementation change to lock statements Implementation changes to field-like events Field-like event access within the declaring class
C# 5	async / await Changes to foreach iteration variable capture Caller information attributes
C# 6	Read-only automatically implemented properties Initializers for automatically implemented properties Remove requirement to call this() in constructors for structs containing automatically implemented properties Expression-bodied members Interpolated string literals The nameof operator The using static directive Object initializers using indexers Collection initializers using extension Add methods The null conditional ?. operator Exception filters Removed restrictions on awaiting in try/catch, try/finally, and try/catch statements
C# 7.0	Tuples Deconstruction via Deconstruct methods Initial patterns: constant patterns, type patterns, var **patterns Use of patterns with the is **operator Use of patterns in switch statements, including guard clauses (when) Ref locals Ref return Binary integer literals Underscore separators in numeric literals Returning custom task types from async methods More kinds of expression-bodied members
C# 7.1	The default **literal Improvements to type patterns matching against generic values Async entry points (async Task Main) Inferred tuple element names
C# 7.2	Allow the conditional ?: operator to work with ref ref readonly locals and return types in parameters Read-only structs Extension methods with ref/in parameters Ref-like structs stackalloc support for Span<T> Nontrailing named arguments The private protected access modifier Underscore separators in numeric literals directly after the 0x or 0b base specifier
C# 7.3	Access to fixed-sized buffers via fields without fixed statements *==* and != operators for tuples Use of pattern and out variables in field, property, and constructor initializers Reassignment of ref locals Initializers in stackalloc statements Pattern-based fixed statements using GetPinnableReference Generic type constraints now permitted on Enum and Delegate New generic type constraint of unmanaged Placement of attributes of fields backing automatically implemented properties
C# 8.0	Readonly members Default interface methods Pattern matching enhancements: Switch expressions, Property patterns, Tuple patterns Positional patterns Using declarations Static local functions Disposable ref structs Nullable reference types Asynchronous streams Indices and ranges Null-coalescing assignment Unmanaged constructed types stackalloc in nested expressions Enhancement of interpolated verbatim string
C# 9.0 (preview)	Init-only properties - Init accessors and readonly fields Records (data class) - With-expressions - Value-based equality - Data members - Positional records - With-expressions and inheritance - Value-based equality and inheritance Top-level programs (omitting Main method) Improved pattern matching - Simple type patterns - Relational patterns - Logical patterns - Target-typed new expressions - Target typed ?? and ?: Covariant returns

참고

• C# in Depth 4 ed
• The history of C# https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-version-history
• Welcome to C# 9.0 https://devblogs.microsoft.com/dotnet/welcome-to-c-9-0/
• dotnet/csharplang Milestone: 9.0 candidate https://github.com/dotnet/csharplang/milestone/15