C# language version history:

These are the versions of C# known about at the time of this writing:

• C# 1.0 released with .NET 1.0 and VS2002 (January 2002)
• C# 1.2 (bizarrely enough); released with .NET 1.1 and VS2003 (April 2003). First version to call Dispose on IEnumerators which implemented IDisposable. A few other small features.
• C# 2.0 released with .NET 2.0 and VS2005 (November 2005). Major new features: generics, anonymous methods, nullable types, and iterator blocks
• C# 3.0 released with .NET 3.5 and VS2008 (November 2007). Major new features: lambda expressions, extension methods, expression trees, anonymous types, implicit typing (var), and query expressions
• C# 4.0 released with .NET 4 and VS2010 (April 2010). Major new features: late binding (dynamic), delegate and interface generic variance, more COM support, named arguments, tuple data type and optional parameters
• C# 5.0 released with .NET 4.5 and VS2012 (August 2012). Major features: async programming, and caller info attributes. Breaking change: loop variable closure.
• C# 6.0 released with .NET 4.6 and VS2015 (July 2015). Implemented by Roslyn. Features: initializers for automatically implemented properties, using directives to import static members, exception filters, element initializers, await in catch and finally, extension Add methods in collection initializers.
• C# 7.0 released with .NET 4.7 and VS2017 (March 2017). Major new features: tuples, ref locals and ref return, pattern matching (including pattern-based switch statements), inline out parameter declarations, local functions, binary literals, digit separators, and arbitrary async returns.
• C# 7.1 released with VS2017 v15.3 (August 2017). New features: async main, tuple member name inference, default expression, and pattern matching with generics.
• C# 7.2 released with VS2017 v15.5 (November 2017). New features: private protected access modifier, Span<T>, aka interior pointer, aka stackonly struct, and everything else.
• C# 7.3 released with VS2017 v15.7 (May 2018). New features: enum, delegate and unmanaged generic type constraints. ref reassignment. Unsafe improvements: stackalloc initialization, unpinned indexed fixed buffers, custom fixed statements. Improved overloading resolution. Expression variables in initializers and queries. == and != defined for tuples. Auto-properties' backing fields can now be targeted by attributes.
• C# 8.0 released with .NET Core 3.0 and VS2019 v16.3 (September 2019). Major new features: nullable reference-types, asynchronous streams, indices and ranges, readonly members, using declarations, default interface methods, static local functions, and enhancement of interpolated verbatim strings.
• C# 9.0 released with .NET 5.0 and VS2019 v16.8 (November 2020). Major new features: init-only properties, records, with-expressions, data classes, positional records, top-level programs, improved pattern matching (simple type patterns, relational patterns, logical patterns), improved target typing (target-type new expressions, target typed ?? and ?), and covariant returns. Minor features: relax ordering of ref and partial modifiers, parameter null checking, lambda discard parameters, native ints, attributes on local functions, function pointers, static lambdas, extension GetEnumerator, module initializers, and extending partial.
• C# 10.0 released with .NET 6.0 (November 2021). Major new features: record structs, struct parameterless constructors, interpolated string handlers, global using directives, file-scoped namespace declarations, extended property patterns, const interpolated strings, mixed assignment and declaration in deconstruction, async method builders (via attributes) for individual methods, the CallerArgumentExpression attribute for parameters, enhanced #line pragmas.
• C# 11.0 released with .NET 7.0 (November 2022). Major new features: file-scoped types, generic math support, auto-default structs, pattern match Span<char> on a constant string, extended nameof scope, numeric IntPtr, UTF-8 string literals, required members, ref fields and scoped ref, raw string literals, improved method group conversion to delegate, warning wave 7, generic attributes, newlines in string interpolation expressions, list patterns

In response to the OP's question:

What are the correct version numbers for C#? What came out when? Why can't I find any answers about C# 3.5?

There is no such thing as C# 3.5 - the cause of confusion here is that the C# 3.0 is present in .NET 3.5. The language and framework are versioned independently, however - as is the CLR, which is at version 2.0 for .NET 2.0 through 3.5, .NET 4 introducing CLR 4.0, service packs notwithstanding. The CLR in .NET 4.5 has various improvements, but the versioning is unclear: in some places it may be referred to as CLR 4.5 (this MSDN page used to refer to it that way, for example), but the Environment.Version property still reports 4.0.xxx.

As of May 3, 2017, the C# Language Team created a history of C# versions and features on their GitHub repository: Features Added in C# Language Versions. There is also a page that tracks upcoming and recently implemented language features.

This is the same as most answers here, but tabularized for ease, and it has Visual Studio and .NET versions for completeness.

Versions since .NET Core

* - .NET 5.0 is not a newer version of .NET framework but .NET Core 3. Starting from .NET 5.0, there are no newer versions of .NET full framework.

** - There are no separate CLR (CoreCLR) versions for .NET Core. Whatever is the .NET Core version is the CoreCLR version. So not mentioning it.

Note: .NET is pretty much independent of VS and compiler versions, there is no correlation between versions of each. The above lists don't imply certain version of .NET are tied to certain C# or VS versions. This is just compiled in their chronological order of their releases (VS/C# and .NET). Some releases happen to be together, hence they sit in the same row.
Refer to ".NET Framework versions and dependencies" and ".NET release cadence" for more.

The biggest problem when dealing with C#'s version numbers is the fact that it is not tied to a version of the .NET Framework, which it appears to be due to the synchronized releases between Visual Studio and the .NET Framework.

The version of C# is actually bound to the compiler, not the framework. For instance, in Visual Studio 2008 you can write C# 3.0 and target .NET Framework 2.0, 3.0 and 3.5. The C# 3.0 nomenclature describes the version of the code syntax and supported features in the same way that ANSI C89, C90, and C99 describe the code syntax and features for C.

Take a look at Mono, and you will see that Mono 2.0 (mostly implemented version 2.0 of the .NET Framework from the ECMA specifications) supports the C# 3.0 syntax and features.

• C# 1.0 with Visual Studio .NET

• C# 2.0 with Visual Studio 2005

• C# 3.0 with Visual Studio 2008

• C# 4.0 with Visual Studio 2010

• C# 5.0 with Visual Studio 2012

• C# 6.0 with Visual Studio 2015

• C# 7.0 with Visual Studio 2017

• C# 8.0 with Visual Studio 2019

• C# 9.0 with Visual Studio 2019

I've summarised most of the versions in this table. The only ones missing should be ASP.NET Core versions. I've also added different versions of ASP.NET MVC.

Note that ASP.NET 5 has been rebranded as ASP.NET Core 1.0 and ASP.NET MVC 6 has been rebranded as ASP.NET Core MVC 1.0.0. I believe this change occurred sometime around Jan 2016.

I have included the release date of ASP.NET 5 RC1 in the table, but I've yet to include ASP.NET core 1.0 and other core versions, because I couldn't find the exact release dates. You can read more about the release dates regarding ASP.NET Core here: When is ASP.NET Core 1.0 (ASP.NET 5 / vNext) scheduled for release?

You can check the latest C# versions here

Comparing the MSDN articles "What's New in the C# 2.0 Language and Compiler" and "What's New in Visual C# 2005", it is possible to deduce that "C# major_version.minor_version" is coined according to the compiler's version numbering.

There is C# 1.2 corresponding to .NET 1.1 and VS 2003 and also named as Visual C# .NET 2003.

But further on Microsoft stopped to increment the minor version (after the dot) numbers or to have them other than zero, 0. Though it should be noted that C# corresponding to .NET 3.5 is named in msdn.microsoft.com as "Visual C# 2008 Service Pack 1".

There are two parallel namings: By major .NET/compiler version numbering and by Visual Studio numbering.

C# 2.0 is a synonym for Visual C# 2005

C# 3.0 corresponds (or, more correctly, can target) to:

• .NET 2.0 <==> Visual C# 2005
• .NET3.0 <==> Visual C# 2008
• .NET 3.5 <==> Visual C# 2008 Service Pack 1

C# 1.0 - Visual Studio .NET 2002

• Classes
• Structs
• Interfaces
• Events
• Properties
• Delegates
• Expressions
• Statements
• Attributes
• Literals

C# 1.2 - Visual Studio .NET 2003

• Dispose in foreach
• foreach over string specialization
• C# 2 - Visual Studio 2005
• Generics
• Partial types
• Anonymous methods
• Iterators
• Nullable types
• Getter/setter separate accessibility
• Method group conversions (delegates)
• Static classes
• Delegate inference

C# 3 - Visual Studio 2008

• Implicitly typed local variables
• Object and collection initializers
• Auto-Implemented properties
• Anonymous types
• Extension methods
• Query expressions
• Lambda expression
• Expression trees
• Partial methods

C# 4 - Visual Studio 2010

• Dynamic binding
• Named and optional arguments
• Co- and Contra-variance for generic delegates and interfaces
• Embedded interop types ("NoPIA")

C# 5 - Visual Studio 2012

• Asynchronous methods
• Caller info attributes

C# 6 - Visual Studio 2015

• Draft Specification online
• Compiler-as-a-service (Roslyn)
• Import of static type members into namespace
• Exception filters
• Await in catch/finally blocks
• Auto property initializers
• Default values for getter-only properties
• Expression-bodied members
• Null propagator (null-conditional operator, succinct null checking)
• String interpolation
• nameof operator
• Dictionary initializer

C# 7.0 - Visual Studio 2017

• Out variables
• Pattern matching
• Tuples
• Deconstruction
• Discards
• Local Functions
• Binary Literals
• Digit Separators
• Ref returns and locals
• Generalized async return types
• More expression-bodied members
• Throw expressions

C# 7.1 - Visual Studio 2017 version 15.3

• Async main
• Default expressions
• Reference assemblies
• Inferred tuple element names
• Pattern-matching with generics

C# 7.2 - Visual Studio 2017 version 15.5

• Span and ref-like types
• In parameters and readonly references
• Ref conditional
• Non-trailing named arguments
• Private protected accessibility
• Digit separator after base specifier

C# 7.3 - Visual Studio 2017 version 15.7

• System.Enum, System.Delegate and unmanaged constraints.
• Ref local re-assignment: Ref locals and ref parameters can now be reassigned with the ref assignment operator (= ref).
• Stackalloc initializers: Stack-allocated arrays can now be initialized, e.g. Span x = stackalloc[] { 1, 2, 3 };.
• Indexing movable fixed buffers: Fixed buffers can be indexed into without first being pinned.
• Custom fixed statement: Types that implement a suitable GetPinnableReference can be used in a fixed statement.
• Improved overload candidates: Some overload resolution candidates can be ruled out early, thus reducing ambiguities.
• Expression variables in initializers and queries: Expression variables like out var and pattern variables are allowed in field initializers, constructor initializers and LINQ queries.
• Tuple comparison: Tuples can now be compared with == and !=.
• Attributes on backing fields: Allows [field: …] attributes on an auto-implemented property to target its backing field.

C# 8.0 - .NET Core 3.0 and Visual Studio 2019 version 16.3

• Nullable reference types: express nullability intent on reference types with ?, notnull constraint and annotations attributes in APIs, the compiler will use those to try and detect possible null values being dereferenced or passed to unsuitable APIs.
• Default interface members: interfaces can now have members with default implementations, as well as static/private/protected/internal members except for state (ie. no fields).
• Recursive patterns: positional and property patterns allow testing deeper into an object, and switch expressions allow for testing multiple patterns and producing corresponding results in a compact fashion.
• Async streams: await foreach and await using allow for asynchronous enumeration and disposal of IAsyncEnumerable collections and IAsyncDisposable resources, and async-iterator methods allow convenient implementation of such asynchronous streams.
• Enhanced using: a using declaration is added with an implicit scope and using statements and declarations allow disposal of ref structs using a pattern.
• Ranges and indexes: the i..j syntax allows constructing System.Range instances, the ^k syntax allows constructing System.Index instances, and those can be used to index/slice collections.
• Null-coalescing assignment: ??= allows conditionally assigning when the value is null.
• Static local functions: local functions modified with static cannot capture this or local variables, and local function parameters now shadow locals in parent scopes.
• Unmanaged generic structs: generic struct types that only have unmanaged fields are now considered unmanaged (ie. they satisfy the unmanaged constraint).
• Readonly members: individual members can now be marked as readonly to indicate and enforce that they do not modify instance state.
• Stackalloc in nested contexts: stackalloc expressions are now allowed in more expression contexts.
• Alternative interpolated verbatim strings: @$"..." strings are recognized as interpolated verbatim strings just like $@"...".
• Obsolete on property accessors: property accessors can now be individually marked as obsolete.
• Permit t is null on unconstrained type parameter

[Source]: https://github.com/dotnet/csharplang/blob/master/Language-Version-History.md

Version     .NET Framework     Visual Studio     Important Features

C# 1.0 .NET Framework 1.0/1.1 Visual Studio .NET 2002

• Basic features

C# 2.0 .NET Framework 2.0 Visual Studio 2005

• Generics
• Partial types
• Anonymous methods
• Iterators
• Nullable types
• Private setters (properties)
• Method group conversions (delegates)
• Covariance and Contra-variance
• Static classes

C# 3.0 .NET Framework 3.0\3.5 Visual Studio 2008

• Implicitly typed local variables
• Object and collection initializers
• Auto-Implemented properties
• Anonymous types
• Extension methods
• Query expressions
• Lambda expressions
• Expression trees
• Partial Methods

C# 4.0 .NET Framework 4.0 Visual Studio 2010

• Dynamic binding (late binding)
• Named and optional arguments
• Generic co- and contravariance
• Embedded interop types

C# 5.0 .NET Framework 4.5 Visual Studio 2012/2013

• Async features
• Caller information

C# 6.0 .NET Framework 4.6 Visual Studio 2013/2015

• Expression Bodied Methods
• Auto-property initializer
• nameof Expression
• Primary constructor
• Await in catch block
• Exception Filter
• String Interpolation

C# 7.0 .NET Core 2.0 Visual Studio 2017

• out variables
• Tuples
• Discards
• Pattern Matching
• Local functions
• Generalized async return types
• Numeric literal syntax improvements

C# 8.0 .NET Core 3.0 Visual Studio 2019

• 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
• Asynchronous disposable
• Indices and ranges
• Null-coalescing assignment
• Unmanaged constructed types
• Stackalloc in nested expressions
• Enhancement of interpolated verbatim strings

C# 8.0 is the latest version of C#. It is supported only on .NET Core 3.x and newer versions. Many of the newest features require library and runtime features introduced in .NET Core 3.x.

The following table lists the target framework with version and their default C# version.

Source - C# language versioning

I was looking for a concise history of the .NET, C#, CLR, and Visual Studio versions alongside the key language features.

Since I couldn't find any up-to-date table that contains all the information I needed in one place - I merged details from the Microsoft docs into what I tried to keep a concise table that contains what I was looking for.

It’s available here: https://mantinband.github.io/dotnet-shmotnet/

I probably have some mistakes or missing information so please feel free to open an issue or contribute over here: https://github.com/mantinband/dotnet-shmotnet

Sneak peek:

You can find a list of all the C# language versions on Microsoft's reference.