Refactors existing C# and VB.NET code to C# 14 and .NET 10 idioms.
npx skills add https://github.com/dotnet/dotnet --skill code-modernization
Please help me install this Agent Skill. Skill: code-modernization Source: https://github.com/dotnet/dotnet/tree/main/src/winforms/.github/skills/code-modernization Command: npx skills add https://github.com/dotnet/dotnet --skill code-modernization
Legacy C# and VB.NET codebases accumulate outdated syntax, inconsistent null handling, and verbose patterns that increase maintenance cost. This Skill provides a rule-based checklist for safely modernizing existing source files to C# 14 / .NET 10 idioms without changing runtime semantics.
== null with is null, applies pattern matching, switch expressions, collection expressions, target-typed new(), and the C# 14 field keyword.var versus explicit types, including casts, generic method calls, and tuple deconstruction.ArgumentNullException.ThrowIfNull, convert event handlers to nullable signatures, and wrap long dot-chains — while skipping multi-targeted files where modern syntax would break older frameworks.Ask the AI to modernize and refactor a specific existing C# file in this repository following the code-modernization rules.
High-intent search queries and answers about installing and using this skill.
Apply safe transformations such as file-scoped namespaces, `is null` pattern matching, switch expressions, collection expressions, target-typed `new()`, and the `field` keyword. Only make changes that cannot alter runtime semantics, and verify the project's target framework first.
Never use `var` for primitive types like int or string. Use `var` when the type is visible on the same line, such as casts, generic method calls with explicit type arguments, or methods whose name implies the return type. Spell out the type when context does not reveal it.
Only if the whole project already processes nullable annotations or the file has an explicit `#nullable enable` directive. Do not add `#nullable enable` to files that lack it, since incremental annotation in a non-NRT-aware codebase risks introducing nullability bugs.
The `is null` pattern-matching form cannot be overridden by custom `==` or `!=` operators, making null checks reliable and consistent. It is the preferred idiom in modern C# codebases and avoids subtle bugs from overloaded equality operators.
Transformations must never change runtime semantics; for example, `public Foo Bar => new Foo()` creates an instance per access while a property initializer creates one. Multi-targeted files and code under eng/common/ must be left untouched, and uncertain changes should get a TODO comment instead.
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