CSharp.Refactor MCP server

CSharp.Refactor

The C# little-sister of FSharp.Refactor

Functional refactoring suggestions for C#, implemented in F# on Roslyn. The rules care about correctness, measured performance and clear idiomatic C# — and deliberately not about naming, layout or conventions.

• light bulb quick fixes in your editor (Visual Studio, VS Code, Rider, anything that hosts Roslyn analyzers)
• a command-line tool that applies them in bulk, build-verified

Suggestions are Info severity: they mark an opportunity, not a defect, and never gate your build. The priority rules, the ones that point at likely defects, are warnings: see Severity for a project that treats warnings as errors.

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Using it

The tool is fully offline: AI has been used to make it, but it doesn't use AI.

Quick start

Nothing to configure — the tool reads your project, reports what it would change, and only edits when you tell it to. The tool runs on the .NET 10 SDK; the projects it works on can target any framework:

dotnet tool install --global csharp-refactor
csharp-refactor Your.csproj --dry-run --api-changes

That prints every fix it would make, with file and position, and writes nothing. When the list looks right, drop the flag to apply them:

csharp-refactor Your.csproj --api-changes

If you don't want change public methods / API, drop --api-changes:

csharp-refactor Your.csproj

It refuses a compilation that does not already build, recompiles in memory after every pass, and builds the project for real at the end, putting the fixes back if that fails.

For light bulbs while you type, see VS Code and Visual Studio IDE-plugin instructions below.

For agentic scenarios the dotnet tool supports MCP.

Using the tool:

Point it at whatever you have — the kind is read off the path:

Your.csproj	one project
Thing.cs	one source file — its project is found and analysed, but only that file is edited
Your.sln, Your.slnx	every C# project the solution lists
src/	the solution in that directory, or the projects beneath it
"src/**/*.csproj"	everything the glob matches
build.csx	one C# script — its own compilation (#r, #load, the NuGet cache), no MSBuild at all; a directory picks its loose scripts up too
C:\git	a workspace of checkouts: each analysed on its own into one report

Editor and CI setup

The analyzers ship as CSharp.Refactor.Analyzers, an ordinary Roslyn analyzer package. Reference it from the project you want analysed and every host — dotnet build, Visual Studio, VS Code's C# extension, Rider — loads it by itself:

<PackageReference Include="CSharp.Refactor.Analyzers" Version="*" PrivateAssets="all" />

The package is a development dependency with no dependencies of its own: it only produces hints and quick fixes, and nothing from it reaches your compiled output or your bin.

The analyzers are built against Roslyn 4.14, so they need Visual Studio 17.14 or the .NET SDK 9.0.300, or newer. An older compiler cannot load them and says so with warning CS9057.

Severity

Most rules are Info. The priority rules (the Priority column of Rules.md) are Warning, because what they find is usually a defect: a lock on this, a disposable field never disposed, an exception rethrown without its cause. In a project with TreatWarningsAsErrors a warning fails the build, the day the package is added. To keep them as warnings there:

<WarningsNotAsErrors>$(WarningsNotAsErrors);CR0047;CR0061</WarningsNotAsErrors>

or lower one in .editorconfig:

dotnet_diagnostic.CR0047.severity = suggestion

Visual Studio 2022 / 2026

Install the CSharp.Refactor extension: every C# project opened in the IDE gets the suggestions and Ctrl+. fixes, with no project change and no effect on builds. Its Tools > CSharp.Refactor menu runs the csharp-refactor tool on the open solution (apply or dry run, with or without --api-changes), writes the SARIF report, opens the notes page and the .editorconfig block — the VS Code extension's commands, in Visual Studio. Or use the package reference above; both work, and a project with both sees each finding once.

VS Code

Install the CSharp.Refactor extension. It bundles the analyzers and, with your consent, wires them into every C# project on the machine through MSBuild's user ImportBefore hook (one props file under your profile, undone by a command). Or use the package reference.

CI

The analyzers run inside the compiler, so a build can write the findings as SARIF with no extra tool:

dotnet build -p:ErrorLog=findings.sarif

The tool's own report carries more — fixes as suggested changes, context snippets, stable fingerprints — and pairs with --dry-run for a lint gate:

      - run: dotnet tool install --global csharp-refactor
      - run: csharp-refactor src/Your.csproj --dry-run --report findings.sarif
      - uses: github/codeql-action/upload-sarif@v3
        if: always()
        with:
          sarif_file: findings.sarif
          category: csharp-refactor

A dry run exits 0 whether or not it found anything; --fail-on-findings makes it exit 3 when a finding survives the filters, and --baseline turns that into a ratchet: findings already in an earlier report are neither reported nor fixed, only new ones surface.

Applying fixes from the command line

csharp-refactor Your.csproj [--dry-run] [--codes CR0090,CR0103] [--categories correctness,performance] [--api-changes]

Flag
--dry-run	Report only: lists every fix it would make, writes nothing.
--codes CR0090,CR0103	Restrict the run to chosen rules; the other rules do not run at all. Naming a rule is an ask: it outranks the rule's default-off status and a config none.
--categories <list>	Restrict to kinds of rule: correctness, performance, idiom, cosmetic. The rules of the other kinds do not run. For a repository you do not maintain, correctness,performance is the set worth a pull request.
--define <symbols>	Preprocessor symbols the run defines, like csc -define: or a DefineConstants entry: repeatable, or ;/,-separated (--define:A and -d:A work too). Added to each project's own DefineConstants — DEBUG, TRACE and the project's constants stay. See Code under #if.
--api-changes	Also apply fixes that change internal or public signatures and shapes, rewriting call sites across the solution (the reference oracle finds them in every project; a caller in a VB project holds the fix, an F# consumer holds the surface). Held back and counted without it.
--report <file>	Write every finding: .sarif, .html (a self-contained page) or .csv.
--baseline <sarif>	The ratchet: findings whose fingerprints appear in this earlier report are neither reported nor fixed.
--fail-on-findings	Exit 3 when any finding survives the filters. Exit contract: 0 clean, 1 failure, 2 usage, 3 findings.
--notes [on|off|only]	List fix-less advisory notes inline; only is the review pass, nothing written. By default a run prints its fixes and one per-category note count; reports and JSON always carry the notes.
--format json	Machine-readable stdout; progress prose moves to stderr.
--rules	Print the rule catalog.
--create-config	Append a commented block of every rule and key at its default to the directory's .editorconfig.
--mcp	Serve analyze and list_rules as an MCP server over stdio. The process and MSBuild stay warm across calls; each call loads its target afresh. The server works inside one directory: csharp-refactor <directory> --mcp, or the directory it is started in. A call's target must lie inside it (a relative target is relative to it), and an apply writes nothing outside it: a run whose fixes reach a file outside, such as a linked source, is put back whole and reported. --mcp --dry-run is a server that applies nothing; --define and --jobs hold for every call, and any other option is refused, since it would be ignored. Analysing a project still loads it through MSBuild, which runs its build logic, so the directory should be a tree you trust.
--parse-only	For a project whose references do not resolve: only the syntax-only rules offer fixes, and the gate is the parse-error count. The project still loads through MSBuild, and the typed rules still report, without a fix. Not a substitute for a real run.
--max-passes <n>	Fix-then-reanalyse iterations (default 5). A pass after the first re-analyses only the files the previous pass edited or held a fix in.

Projects are loaded through MSBuildWorkspace — SDK-style projects of any target, net48 included, in every framework flavour they list. A legacy project (<Project ToolsVersion="4.0" …> with explicit <Compile> items) is read from its project file when the .NET Framework build host does not answer: compile items, defines, HintPath references that exist, the targeting pack's assemblies and project references. Without the build tree (a reference a custom .targets adds, a sibling bin) some types stay unresolved; the run says how many, the typed rules stand down where a type is unknown, the in-memory check holds the error count, and the final build runs through Visual Studio's MSBuild.exe where a baseline build succeeds — otherwise the run is verified in memory only, and says so. An executable another project of the solution references (its tests) is not a leaf: its public shape stays unless --api-changes says otherwise, and every dependent of a changed project is built too. A C# script (.csx) is a compilation of its own: the file parsed as a script, the running runtime as its references, #r resolved against the script's directory, the runtime and the NuGet cache (#r "nuget: Name, Version"), #load read by the compiler for context (a loaded file is edited when it is swept as a target of its own, and a directory sweeps every loose script). Nothing builds a script, so the run is verified in memory: the error count may not grow, and a script host's globals (Args) count as errors on both sides.

Code under #if LOCAL_BUILD

Code behind a preprocessor symbol nothing defines is not in the parse tree at all: the tool neither analyses it nor sees what a fix elsewhere does to it, and the verification build compiles without it too. A repository that keeps, say, a local-development path under #if LOCAL_BUILD names the symbol for the run:

csharp-refactor Your.sln --define LOCAL_BUILD        # or --define:LOCAL_BUILD, -d:LOCAL_BUILD
csharp-refactor Your.sln --define "LOCAL_BUILD;CI"   # several at once

or once for every run, in .editorconfig (csharp_refactor.defines = LOCAL_BUILD, below). The run uses both, and says at the start which symbols are active and where each came from. The symbols are added to what each project defines, exactly as dotnet build would see them with DefineConstants set in the environment: the SDK's DEBUG/TRACE, the target framework's NET8_0_OR_GREATER and the project's own $(DefineConstants);FOO all stay. (-p:DefineConstants=LOCAL_BUILD would not do: a global property replaces every one of them.) Every MSBuild the run starts — the workspace's design-time build, restores, the verification builds — gets them that way, and the loaded C# projects, legacy projects and .csx scripts get them in their parse options as well, so a project that assigns DefineConstants outright is still analysed with them. A script whose #r cannot be resolved while another #r sits under #if LOCAL_BUILD gets the hint to pass --define LOCAL_BUILD.

Configuration

Rules are configured the way every Roslyn analyzer is, in .editorconfig, nearest section winning:

[*.cs]
dotnet_diagnostic.CR0103.severity = none          # off
dotnet_diagnostic.CR0006.severity = suggestion    # a default-off rule, on
csharp_refactor.CR0006.then_at_least = 30         # a rule's knob

csharp_refactor.public_api   = false              # nothing links to this assembly
csharp_refactor.api_changes  = true               # --api-changes as a standing decision
csharp_refactor.suppressions = no-correctness     # all | no-correctness | none
csharp_refactor.skip_microsoft_duplicates = true  # the Microsoft analyzers run here: skip what they already report
csharp_refactor.ignore_paths = generated;external/imported
csharp_refactor.defines = LOCAL_BUILD             # like --define on every run

csharp_refactor.defines is a run-level key: it is read once per run, from the .editorconfig nearest the target (the walk stops at root = true), and adds to --define, never replacing it.

csharp-refactor --create-config writes the block for you, every value at this build's default, so it changes nothing until you edit a line. Each rule's knobs follow its severity line, at their defaults; a knob whose default is not one value (CR0023's measured floor, a rule's wrap_column, which falls back to the file's max_line_length) is written commented out.

A generated file is never touched, in the editor or by the tool: one under bin, obj, node_modules or .git, one named *.g.cs, *.designer.cs, *.generated.cs or *.g.i.cs, one listed in ignore_paths, and one whose header says <auto-generated> (a T4 template's, a resx designer's, the SDK's AssemblyInfo) or "This code was generated by".

Individual findings are silenced with Roslyn's own means — #pragma warning disable CR0090, [SuppressMessage], a severity of none — and editors honour those natively. The suppressions policy governs the tool: no-correctness reports a suppressed correctness finding anyway (and never auto-fixes it), none reports every suppressed finding, --honor-suppressions overrides either for one run. The run summary counts what suppressions silenced.

A rule that shadows a Microsoft analyzer rule (CA2213, CA1031, CA2000 …) reports and fixes its shapes by default: a severity line does not prove the Microsoft analyzer is loaded. With csharp_refactor.skip_microsoft_duplicates = true it stands down for those shapes wherever the Microsoft rule has a dotnet_diagnostic.<id>.severity line other than none in the file's effective config, so nothing is reported twice and nothing oscillates. Rules.md names each rule's twins.

Warnings as errors: The tool uses the same settings as your CI build (TreatWarningsAsErrors, WarningsAsErrors, .editorconfig severities). If a fix would trigger an analyzer error, the tool leaves that file alone, names the analyzer and the rule, and tells you what to change. Use to take those fixes anyway (the warning stays visible), or to silence the analyzer for everyone.

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Improving it

Building and testing

dotnet build
dotnet test

Before committing:

dotnet tool restore
dotnet fantomas src tests
dotnet dotnet-fsharplint lint src/CSharp.Refactor.Analyzers/CSharp.Refactor.Analyzers.fsproj

The test harness compiles C# string literals against the running framework, runs the pure rules, applies every fix as text and recompiles; the host tests drive the real DiagnosticAnalyzer and CodeFixProvider through Roslyn; the tool tests sweep a synthetic project through MSBuildWorkspace. tests/run-tests.ps1 runs the lot. benchmarks/PerfClaims holds the before/after pair behind every performance claim.

A multi-line input is an indented block through the csharp helper (tests/CSharp.Refactor.Tests/TestSource.fs): the closing quotes' indentation is cut from every line, so the rule sees the class at column 0, and Fantomas can move the block without changing it.

let source =
    csharp
        """
        class C
        {
            int F(int x) => x + 1;
        }
        """

fires 3 "CR0046" source returns a rule's suggestions and asserts their count, and assertFired does the same for a list filtered by hand. A mismatch lists the text each suggestion fired on, so the failure names the site that was missed, where a bare count would not.

A literal that must be a constant (an attribute argument, a [<Literal>], a printf format) or holds a tab, a \r or trailing whitespace stays an escaped "...\n..." string.

Beside the example-based suite, tests/CSharp.Refactor.PropertyTests is an FsCheck suite over generated programs: a class of members each shaped for a rule, and method bodies around boolean terms over three integers, with an interpreter over the Roslyn tree as the oracle. Its properties are the invariants the rules promise - every fix, editor-only alternatives included, keeps the program compiling; no rule throws on a compilation recovered from damaged text; applying fixes one at a time reaches a fixed point; and the boolean and control-flow rewrites (CR0001, CR0005, CR0007, CR0008, CR0011) keep the method's truth table at every step. The generators have a shape for most rules (of those without, CR0017 is subsumed by CR0160 wherever both read a shape, and CR0156 and CR0158 need C# 15, which the harness cannot parse), and a coverage test tries every shape on its own and all of them together: if a shape stops firing the rule it was written for, it says which. Two more properties dress the programs the way real files come — #regions around members, a #if and a comment inside a member, CRLF, tabs, a #pragma — and require every fix to compile and to leave what was there (no swallowed directive or comment, no bare LF in a CRLF file); and compile them at every language version from C# 7.3 to 13, requiring a fix to use no syntax newer than the file's own. Both run deterministically over every shape as well as over random programs. A rewrite that would compile and mean something else is out of the compile invariant's reach, so a shape carries a decoy where it can: the CR0100/CR0101 shape calls a method whose FormattableString overload is [Obsolete(error)], so an interpolation that re-binds the call is a compile error; the CR0082 shape hands one tuple to object, where a retype would not compile. What no decoy can express (a record made of a type with behaviour, a record dumped into an exception message) stays with the example suite.

Design principles

1. Never break user code. A fix is only offered when it is provably safe; the borderline case produces no suggestion. A fix must be right without a compile: the same fix runs in the editor, where nothing verifies it.
2. Guards over removal. An unsafe rewrite gets a typed proof that the shape is safe and stands down where the proof fails; the rule is never deleted or blanket-disabled.
3. Minimal edits. Fixes are range-based text edits; formatting outside the edited range is untouched.
4. Pure core, thin adapters. Each rule is a pure function from a syntax tree and semantic model to suggestions; the Roslyn analyzer, the fix provider and the tool are one-line adapters over it.
5. Hints point toward idiomatic C# only. Reversible matters of taste belong to the IDE's refactorings, not to a diagnostic.

See DESIGN.md for the full safety model, the yields-to gate, the planned catalog and the relationship to Microsoft's analyzers.