Terse syntax C# command line parser for .NET with F# support
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Command Line Parser Library beta for CLR.

The Command Line Parser Library offers CLR applications a clean and concise API for manipulating command line arguments and related tasks, such as defining switches, options and verb commands. It allows you to display a help screen with a high degree of customization and a simple way to report syntax errors to the end user.

Everything that is boring and repetitive about parsing command line arguments is delegated to the library, letting developers concentrate on core logic. It's written in C# and doesn't depend on other packages.

This library provides hassle free command line parsing with a constantly updated API since 2005.


  • .NET Framework 4.0+
  • Mono 2.1+ Profile

Current Release:

  • This is a pre-release, for documentation please read appropriate wiki section. From version 2.0.x-pre+ parsing kernel was rewritten and public API simplified.

At glance:

  • One line parsing using default singleton: CommandLine.Parser.Default.ParseArguments(...).
  • Automatic or one line help screen generator: HelpText.AutoBuild(...).
    • Supports --help, --version, version and help [verb] by default.
  • Map to sequences (IEnumerable<T>) or scalar types, including enum and Nullable<T>.
  • You can also map to every type with a constructor that accepts a string (like System.Uri).
  • Plug-In friendly architecture as explained here.
  • Define verb commands as git commit -a.
  • Unparsing support: CommandLine.Parser.Default.FormatCommandLine<T>(T options).
  • F#-friendly with support for option<'a>, see demo.
  • Most of features applies with a CoC philosophy.
  • C# demo: source here.

To install:

  • NuGet way (latest stable): Install-Package CommandLineParser
  • NuGet way (latest version): Install-Package CommandLineParser -pre
  • XCOPY way: cp -r ClonedRepo/src/CommandLine To/Your/Project/Dir

To build:

  • FAKE Script
  • MS Visual Studio
  • Xamarin Studio

Public API:

Latest changes are recorded from Version, please refer to this document.

Used by:


The project is well suited to be included in your application. If you don't merge it to your project tree, you must reference CommandLine.dll and import CommandLine and CommandLine.Text namespaces (or install via NuGet). The help text builder and support types are in the CommandLine.Text namespace that is loosely coupled with the parser. It is good to know that the HelpText class will avoid a lot of repetitive coding.


Define a class to receive parsed values:

class Options {
  [Option('r', "read", Required = true,
    HelpText = "Input files to be processed.")]
  public IEnumerable<string> InputFiles { get; set; }

  // Omitting long name, default --verbose
    HelpText = "Prints all messages to standard output.")]
  public bool Verbose { get; set; }

  [Option(Default = "中文",
    HelpText = "Content language.")]
  public string Language { get; set; }

  [Value(0, MetaName = "offset",
    HelpText = "File offset.")]
  public long? Offset { get; set; }

Consume them:

static int Main(string[] args) {
  var result = CommandLine.Parser.Default.ParseArguments<Options>(args);
  var exitCode = result
      options = > {
        if (options.Verbose) Console.WriteLine("Filenames: {0}", string.Join(",", options.InputFiles.ToArray()));
        return 0; },
      errors => {
	    return 1; });
  return exitCode;


type options = {
  [<Option('r', "read", Required = true, HelpText = "Input files.")>] files : seq<string>;
  [<Option(HelpText = "Prints all messages to standard output.")>] verbose : bool;
  [<Option(Default = "русский", HelpText = "Content language.")>] language : string;
  [<Value(0, MetaName="offset", HelpText = "File offset.")>] offset : int64 option;

Consume them:

let main argv =
  let result = CommandLine.Parser.Default.ParseArguments<options>(argv)
  match result with
  | :? Parsed<options> as parsed -> run parsed.Value
  | :? NotParsed<options> as notParsed -> fail notParsed.Errors

For verbs:


[Verb("add", HelpText = "Add file contents to the index.")]
class AddOptions {
  //normal options here
[Verb("commit", HelpText = "Record changes to the repository.")]
class CommitOptions {
  //normal options here
[Verb("clone", HelpText = "Clone a repository into a new directory.")]
class CloneOptions {
  //normal options here

int Main(string[] args) {
  return CommandLine.Parser.Default.ParseArguments<AddOptions, CommitOptions, CloneOptions>(args)
      (AddOptions opts) => RunAddAndReturnExitCode(opts),
      (CommitOptions opts) => RunCommitAndReturnExitCode(opts),
      (CloneOptions opts) => RunCloneAndReturnExitCode(opts),
      errs => 1);


open CommandLine

[<Verb("add", HelpText = "Add file contents to the index.")>]
type AddOptions = {
  // normal options here
[<Verb("commit", HelpText = "Record changes to the repository.")>]
type CommitOptions = {
  // normal options here
[<Verb("clone", HelpText = "Clone a repository into a new directory.")>]
type CloneOptions = {
  // normal options here

let main args =
  let result = Parser.Default.ParseArguments<AddOptions, CommitOptions, CloneOptions> args
  match result with
  | :? CommandLine.Parsed<obj> as command ->
    match command.Value with
    | :? AddOptions as opts -> RunAddAndReturnExitCode opts
    | :? CommitOptions as opts -> RunCommitAndReturnExitCode opts
    | :? CloneOptions as opts -> RunCloneAndReturnExitCode opts
  | :? CommandLine.NotParsed<obj> -> 1


Jet Brains ReSharper

Thanks to JetBrains for providing an open source license for ReSharper.

Main Contributors (alphabetical order):

  • Alexander Fast (@mizipzor)
  • Dan Nemec (@nemec)
  • Kevin Moore (@gimmemoore)
  • Steven Evans
  • Thomas Démoulins (@Thilas)

Resources for newcomers:

Latest Changes:

  • Implemented issue #31 (double dash), thanks also to PR #77 by Tom Glastonbury (@tg73).
  • Merged pull request #87, thanks to @randacc.
  • Merged pull request #95, thanks to Dan Nemec (@nemec).
  • Merged pull request #97, thanks to @bolha7 and @nemec.
  • Merged pull request #103, thanks to @krs43.
  • Merged pull request #119, thanks to @andrecarlucci.
  • Added F# demo (as a simple script); removed specific API (work-in-progress) from solution.
  • Merged pull request #144, thanks to @JLRishe.
  • Merged pull request #154, thanks to @txdv.
  • Implemented issue #68 (option list behaviour).
  • Fixed issue #157 (range problems).
  • Fixed issue #159 (min constraint).
  • Fixed issue #160 (max constraint).
  • Fixed issue #161 (min/max constraint for values).
  • Increased test coverage.
  • Fixed issue #149 (valid numeric input for enums).
  • Fixed issue #164 (fixed bug in required value constraint).
  • Important fix on scalar string value adjacent to string sequence (without constraints).
  • Adding more tests for token partitioners.
  • Fix in Sequence.Partition().
  • Sequence.Partition() rewritten.
  • Refactoring for Increase Testability.
  • Little change to allow .NET 4.5 build.
  • Better GetHashCode() implementations.
  • New tests added.
  • Fixing FAKE build script.
  • Issue #172 fixed (Max constraint when Min=Max).
  • Merged PR #171 from @mizipzor.
  • Issue #155 Fixed (fix from @guyzeug).
  • Added support for FSharpOption<T> (if not used no need to distribute FSharp.Core.dll).
  • Disallowed Min=Max=0 for sequences, raising exception.
  • Issue #177 Fixed.
  • Issue #112 Fixed.
  • Support for immutable types.
  • PR #123 by @smbecker Merged.
  • Fixes.
  • Issue #179 Implemented (Value|Option.DefaultValue -> Default).
  • Issue #180 Implemented (better ParserResult<T> model).
  • Issue #181 Implemented.
  • Added Return<TSource,TResult>(...) to ParserResult<TSource>.
  • Issue #183 (option list in verb scenario) Fixed by @Thilas.
  • Issue #183 (immutable in verb scenario) reported by @Thilas Fixed.
  • Default --help command refactored.
  • Added WithParsed<T>() for verbs.
  • Added Return<T1...T16>() for verbs.
  • Automatic --version handling.
  • Added C# demo.
  • Issue #189 Implemented.
  • Issue #190 Fixed/Implemented (--help/--version/version for verbs).
  • Issue #188 (reported by @diversteve) Fixed.
  • Issue #191 (--version/version added to help screen) Fixed.
  • Issue #162 (ValueAttribute handled in help screen) Implemented.
  • PR #197 (by @Thilas) Implemented.
  • Issue #202 (reported by @StevenLiekens) Fixed.
  • Managing deps with Paket.
  • Issue #203 Implemented.
  • Issue #204 (reported by @Kimi-Arthur) Fixed.
  • PR #205 (by @forki) Merged.
  • Refactoring with RailwaySharp.ErrorHandling.
  • Test project refactoring.
  • Issue #186 Implemented: Adding unparse support.
  • PR #207 Merged.
  • Using new Paket-friendly CSharpx and RailwaySharp.
  • Added F# option support to FormatCommandLine().
  • ParserResult<T> internally refactored, minor breaking changes to HelpText.
  • Added FormatCommandLine() overload with settings.
  • Issue #208 Implemented (extended usage text support).
  • Internal/external refactorings.
  • Minor HelpText refactorings.
  • Issue #210 (reported by @mac2000) Implemented.
  • Test refactorings.
  • Fixing XML comments.
  • Changes in usage text handling.
  • Issue #65 (re) Fixed for 2.0.x library.
  • Issue #218 Fixed (IgnoreUnknownArguments).
  • PR #215 (by @Thilas) Merged.
  • PR #216 (by @Thilas) Merged.
  • Issue #217 Fixed.
  • Renaming tests fakes.
  • Issue #220 Implemented.
  • Replacing Either<L,R> with Result<TSucc, TMsg>.
  • Centralizing Tokenizer configuration.
  • Issue #225 (reported by @rmunn) Fixed.
  • Issue #225/b (reported by @rmunn) Fixed.
  • All ParserResult<T>.Return renamed to MapResult.
  • PR #227 (by @Thilas) Merged.
  • Reverted back to Tuple<...> instead of TokePartitions type.
  • PR #227 (by @cfeilen) Merged.
  • PR #231 (by @kshanafelt) Merged.
  • Updated RailwaySharp source reference.
  • Internal refactoring.
  • Added properties test project using FsCheck.
  • Issue #241 (reported by @pvmraghunandan) Fixed.
  • PR #255 fixes Issue #254 by @scadorel Merged.


Giacomo Stelluti Scala

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