Throwaway Roslyn rewriters for structural C# changes

For a mechanical change that's the same shape across many files — strip a set of attributes, swap one API for another, rename a pattern, reshape method signatures — a regex or a Python script is guessing at C# syntax and will mangle edge cases (nested generics, trivia, string literals, conditional compilation). A small Roslyn program parses the actual syntax tree and edits it structurally, so the change is correct and the formatting/trivia survives. It's the reliable way to do predictable bulk edits.

Reach for this when: the change is structural and repeatable, affects many files, and a wrong text-match would silently corrupt code. For one-off edits or genuinely judgment-heavy changes, just edit directly (or drive subagents — see subagent-driven-development).

The lightweight path: a single-file dotnet run app

Since .NET 10 you don't need a project — a single .cs file with directives runs directly. This is the ideal throwaway harness: one file, no scaffolding, delete it when done.

#:package Microsoft.CodeAnalysis.CSharp@5.0.0

using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.CSharp.Syntax;

foreach (var path in Directory.EnumerateFiles(args[0], "*.cs", SearchOption.AllDirectories))
{
    if (path.Contains("/obj/") || path.Contains("/bin/")) continue;

    var text = await File.ReadAllTextAsync(path);
    var tree = CSharpSyntaxTree.ParseText(text);
    var root = await tree.GetRootAsync();

    var newRoot = new MyRewriter().Visit(root);
    if (newRoot == root) continue;                    // nothing changed — skip

    await File.WriteAllTextAsync(path, newRoot.ToFullString());
    Console.WriteLine($"rewrote {path}");
}

// A CSharpSyntaxRewriter visits nodes and returns replacements; unvisited nodes
// (and all their trivia) are preserved exactly.
sealed class MyRewriter : CSharpSyntaxRewriter
{
    public override SyntaxNode? VisitMethodDeclaration(MethodDeclarationSyntax node)
    {
        // e.g. strip specific attributes, returning the edited node.
        return base.VisitMethodDeclaration(node);
    }
}

Run it: dotnet run rewrite.cs -- /path/to/src. Add more #:package lines as needed. Convert to a real project only if it outgrows one file (dotnet project convert rewrite.cs).

Runnable examples

./examples/ has three complete single-file apps you can copy and adapt — a progression from safest to most powerful:

• 01-survey-attributes.cs — read-only reconnaissance. Walks every .cs file and prints the distinct attribute names it finds. Do this first: understand what's actually in the code before you rewrite it. Syntax-only.
• 02-remove-attributes.cs — a syntax-only CSharpSyntaxRewriter that strips a configured set of attributes off methods and rescues their comments. Defaults to a dry run; --write applies. Covers the common case.
• 03-remove-redundant-call.cs — the semantic case: removes a redundant .AsQueryable() only when the receiver is already IQueryable<T>, which you can only know from the semantic model. Loads the solution via MSBuildWorkspace. Also dry-run by default.

Two habits baked into the examples worth keeping: dry-run by default (print what would change, require --write to touch disk), and skip obj//bin/.

When you need semantics: load the workspace

Syntax-only parsing (above) is enough for most attribute/signature/call-shape edits and is far simpler. Step up to a full workspace only when the change needs the semantic model — resolving what a symbol actually binds to, types, overloads, "is this the System.Text.Json one or ours." Load the solution through MSBuildWorkspace:

MSBuildLocator.RegisterDefaults();                    // before any Roslyn MSBuild call
using var workspace = MSBuildWorkspace.Create();
workspace.RegisterWorkspaceFailedHandler(e => Console.Error.WriteLine(e.Diagnostic));
var solution = await workspace.OpenSolutionAsync(slnPath);

foreach (var project in solution.Projects.Where(p => p.Name == "Target"))
foreach (var doc in project.Documents.Where(d =>
             d.FilePath?.EndsWith(".cs") == true &&
             !d.FilePath.Contains("/obj/") && !d.FilePath.Contains("/bin/")))
{
    var model = await doc.GetSemanticModelAsync();     // semantic info lives here
    var root = await doc.GetSyntaxRootAsync();
    // ...rewrite, then File.WriteAllText(doc.FilePath, newRoot.ToFullString());
}

Needs Microsoft.Build.Locator + Microsoft.CodeAnalysis.Workspaces.MSBuild (+ .CSharp.Features for code-fix-style edits). RegisterDefaults() must run before any MSBuild-backed Roslyn call.

Use the latest stable Roslyn

Default to the newest non-preview Microsoft.CodeAnalysis.* (the 5.0.0 line) for these throwaway tools. There's no downside: the harness is disposable, doesn't ship, and never has to match the target repo's compiler version — it just reads/parses .cs text. Newer Roslyn also pulls a patched MSBuild, so the workspace example restores with no vulnerability advisories, whereas the older 4.x line drags in a flagged Microsoft.Build transitively. Pin all the Microsoft.CodeAnalysis.* packages to the same version. Don't reach for preview builds unless you specifically need an unreleased language feature.

Doing the edit right

• Edit the tree, not the text. Use ReplaceNode/ReplaceNodes, WithAttributeLists, SyntaxFactory, or a CSharpSyntaxRewriter. Nodes are immutable — every "change" returns a new node; reassign.
• Preserve trivia. Whitespace, comments, and newlines are trivia attached to tokens. When you remove or replace a node, carry the leading/trailing trivia across (as AttributeRemover does) so you don't leave dangling blank lines or eat comments.
• Bail when nothing changed. if (newRoot == root) continue; — don't rewrite (and reformat/touch) files the transform didn't actually affect.
• Normalize output deliberately. Decide on line endings and trailing whitespace and apply consistently (for example normalize to \n, trim line ends, and write UTF-8 without a BOM). Or run dotnet format afterward instead of hand-normalizing.

After running it

• Build is the verdict. dotnet build + analyzers + tests (see csharp-verification). The rewriter is only correct if the result compiles clean and tests pass — confirm, don't assume.
• Diff before trusting. Skim git diff for a few representative files and any odd cases, and check that the exact spans changed are the ones you intended — a boundary-less text sweep (regex/sed) can splice a comment off its method or rename an identifier inside a string literal, and nothing says so until the build breaks. A structural rewrite is predictable, which makes a wrong assumption show up uniformly — easy to catch in a sample. Rebuild after.
• Delete the harness. It's throwaway tooling — don't commit rewrite.cs into the product repo unless the team wants to keep it. (Keep it in a scratch/tools location if it's worth re-running.)

Anti-patterns

• Regex/sed/Python rewriting C# structure — mangles generics, trivia, strings, and #if blocks; use the parser.
• Mutating .ToFullString() text after parsing instead of editing the tree — throws away the whole point.
• Dropping trivia, leaving blank-line craters or deleting comments.
• Loading a full MSBuildWorkspace when syntax-only parsing would do — slower and more setup for no benefit unless you need the semantic model.
• Forgetting MSBuildLocator.RegisterDefaults() before MSBuild-backed calls (cryptic load failures).
• Declaring success on "it ran" — only dotnet build + tests confirm the rewrite was correct.