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The purpose of this library is to make command line tools first class by providing a command line parser. We've already made an attempt in 2009 but that wasn't a design we (or the community) was happy with.

Here are the goals:

  • Designed for cross-platform usage
  • Lightweight with minimal configuration
  • Optional but built-in support for help, validation, and response files
  • Support for multiple commands, like version control tools

Syntax and API samples are below.

Why a new library?

There is already a set of libraries available for command line parsing, such as:

So the question is: why a new one? There are a couple of reasons:

  1. We want to support a syntax that feels natural when used across platforms. In particular, we want to be very close to the Unix- and GNU style.

  2. We need something that is quite low level. In particular we don't want to have a library that requires reflection for attribute discovery or for setting properties.

  3. We want an experience that achieves an extremely minimal setup in terms of lines of code required for parsing.

While some of the libraries solve some of these aspects none of them solve the combination.

Of course, providing a command line parser isn't just providing a parsing mechanism: in order to be usable, the library has to be opinionated in both the supported syntax as well as in the shape of the APIs. In the BCL, we've always taken the stance that we want to provide a layered experience that allows getting the 80% scenario done, while allowing to be extensible for a potential long tail of additional scenarios. If that means we've to make policy decisions so be it because not making those forces all of our consumers to come up with their own policy.

That being said, the goal isn't to provide the final command line parser library. In fact, I'm not aware of any platform that gets away with having a single one. If you're happy with the one you're already using or if you even wrote your own: that's perfectly fine. But after one and a half decades it's time for the BCL to provide a built-in experience as well.

Work in progress

  • Should we support a case insensitive mode?
  • Should we have a way to name option arguments, e.g. DefineOption("n=name")?
  • Should provide a string based approach to define usage?
  • Should we provide an error handler?
  • Should we provide a help request handler?
  • Should we expose the response file hander?
  • Should we allow "empty" commands, so that the tool can support options without a command, like git --version?


The syntax conventions are heavily inspired by the following existing conventions:

In general, all strings are treated in a case sensitive way. This allows supporting options that only differ by case, which is pretty common on Unix systems, e.g.

# This reverses the output
$ ls -r *.txt
# This does a recursive search
$ ls -R *.txt

Single character options

Single character options are delimited by a single dash, e.g.

$ tool -x -d -f

They can be bundled together, such as

$ tool -xdf

Please note that slashes aren't supported.

Keyword options

Keyword options are delimited by two dashes, such as:

$ tool --verbose

Option arguments

Both, the single letter form, as well as the long forms, support arguments. Arguments must be separated by either a space, an equal sign or a colon:

# All three forms are identical:
$ tool --out result.exe
$ tool --out=result.exe
$ tool --out:result.exe

Multiple spaces are allowed as well:

$ tool --out  result.exe
$ tool --out =   result.exe
$ tool --out : result.exe

This even works when combined with bundling, but in that case only the last option can have an argument. So this:

$ tool -am "hello"

is equivalent to:

$ tool -a -m "hello"

Multiple occurrences

Unix has a strong tradition for scripting. In order to make it easier to forward arguments to scripts, it's common practice to allow options to appear more than once. The semantics are that the last one wins. So this:

$ tool -a this -b -a that

has the same effect as that:

$ tool -b -a that


Parameters, sometimes also called non-option arguments, can be anywhere in the input:

# Both forms equivalent:
$ tool input1.ext input2.ext -o result.ext
$ tool input1.ext -o result.ext input2.ext


Very often, tools have multiple commands with independent options. Good example are version control tools, e.g.

$ tool fetch origin --prune
$ tool commit -m 'Message'

Response files

It's common practice to allow passing command line arguments via response files. This can look as follows:

$ tool -f -r @D:\src\defaults.rsp --additional

The API supports multiple response files being passed in. It will simply expand those in-place, i.e. it's valid to have additional options and parameters before, as well as after the response file reference.

API Samples

Hello world

using System;
using System.CommandLine;

static class Program
    static void Main(string[] args)
        var addressee = "world";

        ArgumentSyntax.Parse(args, syntax =>
            syntax.DefineOption("n|name", ref addressee, "The addressee to greet");

        Console.WriteLine("Hello {0}!", addressee);

Usage looks as follows:

$ ./hello -h
usage: hello [-n <arg>]

    -n, --name    The addressee to greet

$ ./hello
Hello world!
$ ./hello -n Jennifer
Hello Jennifer!
$ ./hello --name Tom
Hello Tom!
$ ./hello -x
error: invalid option -x

Defining options

The ArgumentSyntax class allows defining options and parameters for any data type. In order to parse the value, you need to supply a Func<string, T> that performs the parsing. So if you want to use a guid, you could define an option like this:

Guid guid = Guid.Empty;
syntax.DefineOption("g|guid", ref guid, Guid.Parse, "The GUID to use");

The library provides overloads that handle the most common types, such as string, int, and bool, so that you don't have to pass in parsers for those.

Boolean options are specially handled in that they are considered flags, i.e. they don't require an argument -- they are simply considered true if they are specified. However you can still explicitly pass in true or false. So this

$ tool -x

Is equivalent to

$ tool -x:true

Normally, options must supply an argument value (or it's considered an error). However, there may be times when you want to allow a particular option whether it supplies a value or not. To do this, use the overloaded DefineOption() methods:

int port = 1234;
var option = syntax.DefineOption("s|server", ref port, false,
    "Start the server, optionally specifying a port");

Then the value in code will be set regardless of whether the command line option specifies one and an error will not be thrown if the option value is omitted. You can check for whether the option was specified at all by checking the Argument.IsSpecified flag. For example, given the above option definition:

$ tool
# port = 1234, option.IsSpecified = false

$ tool --server
# port = 1234, option.IsSpecified = true

$ tool --server 9876
# port = 9876, option.IsSpecified = true

The syntax used to define the option supports multiple names by separating them using a pipe. All names are aliases for the same option. For diagnostic purposes, the first name will be used. By convention that should be the short name, but it's really up to you.

Defining parameters

Parameters work in a very similar way:

Guid guid = Guid.Empty;
syntax.DefineParameter("guid", ref guid, Guid.Parse, "The GUID to use");

However, since parameters are matched by position and not by using a named option, the name is only used for diagnostic purposes and to render a readable syntax. Hence, they don't support the pipe syntax because having multiple names wouldn't make any sense there.

Please note that parameters must be specified after options. The reason being that the parser needs to know all options before it can match parameters. Otherwise parsing this command would be ambiguous:

$ tool -x one two

Without knowing whether -x takes an argument, it's not clear whether one will be an argument or the first parameter.

Defining option and parameter lists

Both, options and parameters, support the notion of lists. For example, consider a compiler comp:

$ comp -D DEBUG -D ARCH=x86 source1 source2 -out:hello

You would define the options and parameters as follows:

string output = string.Empty;
IReadOnlyList<string> defines = Array.Empty<string>();
IReadOnlyList<string> sources = Array.Empty<string>();

syntax.DefineOption("out", ref output, "Output name");
syntax.DefineOptionList("D|define", ref defines, "Preprocessor definitions");
syntax.DefineParameterList("source", ref sources, "The source files to compile");

In general, you can define multiple option lists but only one parameter list. The reason being that a parameter list will consume all remaining parameters. You can define individual parameters and a parameter list but the parameter list must be after the individual parameters otherwise the individual ones will never be matched.

Defining commands

Commands are defined in a similar way to options and parameters. The way they are associated with options and commands is by order:

var command = string.Empty;
var prune = false;
var message = string.Empty;
var amend = false;

syntax.DefineCommand("pull", ref command, "Pull from another repo");
syntax.DefineOption("p|prune", ref prune, "Prune branches");

syntax.DefineCommand("commit", ref command, "Committing changes");
syntax.DefineOption("m|message", ref message, "The message to use");
syntax.DefineOption("amend", ref amend, "Amend existing commit");

In this case the pull command has a -p option and the commit command has -m and --amend options. It's worth noting that you can use the same option name between different commands as they are logically in different scopes.

In order to check which command was used you've two options. You can either use the version we used above in which case the ref variable passed to DefineCommand will contain the name of the command that was specified. But you're not limited to just plain strings. For example, this will work as well:

enum Command { Pull, Commit }

// ...

Command command = Command.Pull;
syntax.DefineCommand("pull", ref command, Command.Pull, "Pull from another repo");
syntax.DefineCommand("commit", ref command, Command.Commit, "Committing changes");

Custom help

By default, ArgumentSyntax will display the help and exit when -?, -h or --help is specified. Some tools perform different actions, for instance git, which displays the help on the command line when -h is used but opens the web browser when --help is used.

You can support this by handling help yourself:

ArgumentSyntax.Parse(args, syntax =>
    // Turn off built-in help processing

    syntax.HandleHelp = false;

    // Define your own options

    syntax.DefineOption("n|name", ref addressee, "The addressee to greet");

    // Define custom help options. Optionally, you can hide those options
    // from the help text.

    var longHelp = syntax.DefineOption("help", false);
    longHelp.IsHidden = true;

    var quickHelp = syntax.DefineOption("h", false);
    quickHelp.IsHidden = true;

    if (longHelp.Value)
        // Open a browser
        var url = "https://en.wikipedia.org/wiki/%22Hello,_World!%22_program";
    else if (quickHelp.Value)
        // Show help text. Even if you disable built-in help processing
        // you can still use the built-in help page generator. Optionally,
        // you can ask it to word wrap it according to some maximum, such
        // as the width of the console window.
        var maxWidth = Console.WindowWidth - 2;
        var helpText = syntax.GetHelpText(maxWidth);

Additional validation

Let's say you want to perform additional validation, such as that supplied arguments point to valid files or that certain options aren't used in combination. You can do this by simply adding a bit of validation code at the end of the Parse method that calls ReportError.

ArgumentSyntax.Parse(args, syntax =>
    syntax.DefineOption("n|name", ref addressee, "The addressee to greet");

    if (addressee.Any(char.IsWhiteSpace))
        syntax.ReportError("addressee cannot contain whitespace");

Usage will look like this:

$ ./hello -n "Immo Landwerth"
error: addressee cannot contain whitespace

Custom error handling

There are cases where you want to use ArgumentSyntax in such a way that user errors shouldn't result in the process being terminated. You can do this by disabling the built-in error handling. In that case, the ReportError method -- and thus the Parse method -- will throw ArgumentSyntaxException.

    ArgumentSyntax.Parse(args, syntax =>
        syntax.HandleErrors = false;
        syntax.DefineOption("n|name", ref addressee);
catch (ArgumentSyntaxException ex)
    Console.WriteLine("Ooops... something didn't go well.");
    return 1;

Accessing options and parameters

The ArgumentSyntax object also provides access to the defined options and parameters. The Parse method returns the used instance, so you can use that to access them. You can either access all of the defined ones or you can access only the ones that are relevant to the currently active command.

static void Main(string[] args)
    var addressee = "world";

    var result = ArgumentSyntax.Parse(args, syntax =>
        syntax.DefineOption("n|name", ref addressee, "The addressee to greet");

        if (addressee.Any(char.IsWhiteSpace))
            syntax.ReportError("adressee cannot contain whitespace");

    foreach (var argument in result.GetActiveArguments())
        if (argument.IsOption)
            var names = string.Join(", ", argument.GetDisplayNames());
            Console.WriteLine("Option {0}", names);
            Console.WriteLine("Parameter {0}", argument.GetDisplayName());

        Console.WriteLine("Help         : {0}", argument.Help);
        Console.WriteLine("IsHidden     : {0}", argument.IsHidden);
        Console.WriteLine("Value        : {0}", argument.Value);
        Console.WriteLine("DefaultValue : {0}", argument.DefaultValue);
        Console.WriteLine("IsSpecified  : {0}", argument.IsSpecified);

Turning off response files

In case you don't want consumers to use response files or you need to process parameters that could be prefixed with an @-sign, you can disable response file expansion:

ArgumentSyntax.Parse(args, syntax =>
    syntax.HandleResponseFiles = false;

    // ...