C# Compiler API — Validate .NET Code & Get Roslyn Diagnostics

Check C# code and get every compiler error and warning back as structured JSON — exact codes, messages, and line/column positions. Full semantic Roslyn analysis, no build artifact, no code execution.

Input:

{ "mode": "snippet", "code": "Consle.WriteLine(\"hi\");" }

Output:

{
  "success": false,
  "errorCount": 1,
  "diagnostics": [
    {
      "id": "CS0103",
      "severity": "error",
      "message": "The name 'Consle' does not exist in the current context",
      "file": "snippet.cs",
      "line": 1,
      "column": 1
    }
  ],
  "checkTimeMs": 412
}

Why AI agents need this

LLMs generate C# they cannot compile. One call answers "does this build?" with exact error locations — enabling the loop that makes generated code reliable: generate → check → fix → check. Diagnostic IDs are raw compiler codes (CS0103, CS8602…), so an agent can look them up, fix them, or suppress them deliberately.

Quick start

API:

curl -X POST "https://api.apify.com/v2/acts/<USERNAME>~dotnet-check/run-sync-get-dataset-items?token=<TOKEN>" \
  -H "Content-Type: application/json" \
  -d '{ "mode": "snippet", "code": "var x = 1 + \"two\";" }'

MCP (agents): add this actor at mcp.apify.com?tools=<USERNAME>/dotnet-check.

Input reference

Field	Type	Default	Notes
mode	snippet | project	snippet	Snippet = in-memory semantic check. Project = full dotnet build with NuGet support.
code	string	—	Source to check (snippet mode). Top-level statements, classes, or full files.
files	array	—	Project mode: { "path", "content" } objects. A .csproj is generated if you don't include one.
targetFramework	string	net8.0	net8.0, net9.0, net6.0, netstandard2.0
languageVersion	string	latest	8–13 or latest
outputKind	string	console	console allows top-level statements; classlib for libraries
nullable	bool	true	Nullable reference type analysis (CS86xx warnings)
treatWarningsAsErrors	bool	false
strict	bool	false	Also runs the emit phase (in memory, discarded) for the rare emit-only diagnostics
packages	string[]	—	Project mode: Id@Version, e.g. Newtonsoft.Json@13.0.3

Output reference

Each run pushes one dataset item: success, errorCount, warningCount, checkTimeMs, toolVersion, chargedEvents (billing transparency), and diagnostics[] — always present, errors first, each with id, severity, message, file, line, column, endLine, endColumn.

Pricing

Event	Price	When
Snippet check	$0.005	Per snippet-mode run — a typical validation costs half a cent
Project check	$0.03	Per project-mode build (includes NuGet restore)
Actor start	$0.001	Per run

Security model

• Your code is never executed. The default path performs semantic analysis only — no assembly is ever produced. Strict mode emits to memory and immediately discards it.
• No user-supplied analyzers or build tasks are ever loaded — these would execute arbitrary code at compile time, so they are excluded by design.
• Sources are discarded after the run. Nothing is logged or persisted beyond your own dataset output.

Limits & good to know

• Snippet source up to 1 MB; project mode up to 100 files / 5 MB / 25 packages.
• Common NuGet packages restore from a prewarmed offline cache; unusual ones fetch from nuget.org and take longer.
• Project mode has a 120 s build timeout.
• Policy limits exit gracefully with a status message — they are never disguised as system errors.

Roadmap

nuget-audit (vulnerability scanning), dotnet-format-check, F#/VB.NET checkers. Found an issue or need a feature? Open an issue on this actor — response time is part of the deal.