Csharp14 Dotnet10 Features

C# 14 and .NET 10: What Is New

Author: Guerth Castro (github.com/GuerthCastro). Licensed under MIT.

Quick reference for the language and platform features introduced in C# 14, which ships with the .NET 10 SDK (released November 2025). Use it to confirm a construct is actually available before suggesting it, and to explain why a modern idiom works the way it does.

This is an original summary of publicly documented features. Verify current behavior against Microsoft Learn before relying on any detail here, since this area moves quickly and the docs are the authority.

When to use this

• Reviewing or writing C# for <TargetFramework>net10.0</TargetFramework> and deciding whether a C# 14 construct is idiomatic in that spot.
• The user asks what changed between an older C# version and C# 14, or between .NET 9 and 10.
• Deciding whether an old workaround, such as a hand written backing field or a static class extension method, can now be simplified.
• Flagging a breaking change during a migration, for example field becoming a contextual keyword.

C# 14 language features

field keyword for auto backed properties

Custom get and set logic without declaring a private backing field. The compiler synthesizes it and you refer to it through the contextual keyword field:

public string? FavoritePrimaryColor
{
    get => field;
    set => field = Validate(value);
}

Breaking change: an existing accessor that declared a local variable literally named field either fails to compile or changes meaning under the C# 14 compiler, because field is now contextual inside accessors. Fix by renaming the local or writing @field.

Extension blocks

Alongside classic extension methods (a static class plus a this parameter), C# 14 adds extension(...) blocks. These declare instance methods, instance properties, static methods, and static properties or operators against a type, including things that were previously impossible such as an extension property:

public static class IEnumerableOfInt32Extensions
{
    extension(IEnumerable<int> source)
    {
        public IEnumerable<int> WhereGreaterThan(int threshold) =>
            source.Where(x => x > threshold);

        public bool IsEmpty => !source.Any();
    }
}

Old style extension methods and new style extension blocks coexist in the same static class, so adopting the new syntax is optional rather than a breaking change.

Null conditional assignment

?. may now appear on the left hand side of an assignment. The assignment happens only when the receiver is non null:

customer?.Age = newAge;

That replaces if (customer is not null) { customer.Age = newAge; }.

Partial members expanded

Through C# 13, partial applied to methods, properties, indexers, and events. C# 14 adds partial events and partial instance constructors. Still not allowed: static constructors, finalizers, overloaded operators, delegates, and enums.

nameof with unbound generic types

nameof(List<>) is now valid and returns "List", instead of requiring a closed type such as nameof(List<string>).

File based apps

A single .cs file can be executed with no project file and no directory structure, which suits scripts, prototypes, and teaching:

dotnet run app.cs

Top level statements are supported. This targets quick scripting, not a replacement for project based applications.

.NET 10 platform and library features

• Noun first dotnet CLI aliases. Microsoft now recommends the noun first form of CLI commands for clearer scripting and documentation. The older verb first forms still work.
• Numeric string ordering. A comparison mode that orders embedded numbers numerically rather than lexicographically, so item2 sorts before item10.
• LINQ LeftJoin and RightJoin. New operators alongside Join and GroupJoin, replacing the old GroupJoin plus DefaultIfEmpty plus SelectMany workaround.
• Asynchronous ZIP archive APIs for reading and writing.
• JSON Patch (RFC 6902) for ASP.NET Core, implemented on System.Text.Json.
• A first party OpenAPI document generator for ASP.NET Core Minimal APIs, usable in place of or alongside Swashbuckle style tooling.
• Central Package Management, managing NuGet versions across a solution through Directory.Packages.props instead of pinning per project.

Scope notes

• This file covers C# 14 and .NET 10 specifically. It is not a cumulative history of the language.
• Feature availability can depend on SDK version and language version settings, not only on the target framework. Check <LangVersion> before assuming a construct compiles.
• For migration mechanics such as project files, CI pipelines, and package feeds, use a migration runbook. This skill answers "does this feature exist and how does it work".