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Record C# 8.0 into language history (#2666)

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Language-Version-History.md
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@ -104,3 +104,19 @@ Features Added in C# Language Versions
- [Expression variables in initializers and queries](https://github.com/dotnet/csharplang/blob/master/proposals/csharp-7.3/expression-variables-in-initializers.md): Expression variables like `out var` and pattern variables are allowed in field initializers, constructor initializers and LINQ queries.
- [Tuple comparison](https://github.com/dotnet/csharplang/blob/master/proposals/csharp-7.3/tuple-equality.md): Tuples can now be compared with `==` and `!=`.
- [Attributes on backing fields](https://github.com/dotnet/csharplang/blob/master/proposals/csharp-7.3/auto-prop-field-attrs.md): Allows `[field: …]` attributes on an auto-implemented property to target its backing field.
# C# 8.0 (Visual Studio 2019 version 16.3, .NET Core 3.0)
- [Nullable reference types](https://github.com/dotnet/csharplang/blob/master/proposals/csharp-8.0/nullable-reference-types-specification.md): 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](https://github.com/dotnet/csharplang/blob/master/proposals/csharp-8.0/default-interface-methods.md): interfaces can now have members with default implementations, as well as static/private/protected/internal members except for state (ie. no fields).
- [Recursive patterns](https://github.com/dotnet/csharplang/blob/master/proposals/csharp-8.0/patterns.md): 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](https://github.com/dotnet/csharplang/blob/master/proposals/csharp-8.0/async-streams.md): `await foreach` and `await using` allow for asynchronous enumeration and disposal of `IAsyncEnumerable<T>` collections and `IAsyncDisposable` resources, and async-iterator methods allow convenient implementation of such asynchronous streams.
- [Enhanced using](https://github.com/dotnet/csharplang/blob/master/proposals/csharp-8.0/using.md): 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](https://github.com/dotnet/csharplang/blob/master/proposals/csharp-8.0/ranges.md): 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](https://github.com/dotnet/csharplang/issues/34): `??=` allows conditionally assigning when the value is null.
- [Static local functions](https://github.com/dotnet/csharplang/issues/1565): local functions modified with `static` cannot capture `this` or local variables, and local function parameters now shadow locals in parent scopes.
- [Unmanaged generic structs](https://github.com/dotnet/csharplang/issues/1744): generic struct types that only have unmanaged fields are now considered unmanaged (ie. they satisfy the `unmanaged` constraint).
- [Readonly members](https://github.com/dotnet/csharplang/issues/1710): individual members can now be marked as `readonly` to indicate and enforce that they do not modify instance state.
- [Stackalloc in nested contexts](https://github.com/dotnet/csharplang/issues/1412): `stackalloc` expressions are now allowed in more expression contexts.
- [Alternative interpolated verbatim strings](https://github.com/dotnet/csharplang/issues/1630): `@$"..."` strings are recognized as interpolated verbatim strings just like `$@"..."`.
- [Obsolete on property accessors](https://github.com/dotnet/csharplang/issues/2152): property accessors can now be individually marked as obsolete.
- [Permit `t is null` on unconstrained type parameter](https://github.com/dotnet/csharplang/issues/1284)