Commander.NET

C# command-line argument parsing and serialization via attributes. Inspired by JCommander.

If you would like to expand this library, please include examples in your PR or issue, of widely-used CLI tools that support your suggested practice or format.

Installation

Install from NuGet.

Usage

First you need to create a non-abstract class, with a public parameterless constructor, which will describe the parameters.

using Commander.NET.Attributes;

class Options
{
	[Parameter("-i", Description = "The ID")]
	public int ID = 42;

	[Parameter("-n", "--name")]
	public string Name;

	[Parameter("-h", "--help", Description = "Print this message and exit.")]
	public bool Help;
}

Then simply parse the command line arguments

string[] args = { "-i", "123", "--name", "john"};

CommanderParser<Options> parser = new CommanderParser<Options>();

Options options = parser.Add(args)
                        .Parse();

Required parameters

You may specify which of the parameters are required by setting the Required property of the attributes.

[Parameter("-r", Required = Required.Yes)]
public int RequiredParameter;

[Parameter("--lorem", Required = Required.No)]
public bool NotRequiredParameter;

[Parameter("--ipsum")]	// Required.Default
public string ThisOneIsRequired;

[Parameter("--dolor")]	// Required.Default
public string ThisOneIsNotRequired = "Because it has a default value";

As seen in the example above, the Required enum can have one of three values:

• Required.Yes
  • This parameter is required and omitting it will throw a ParameterMissingException during parsing.
• Required.No
  • This parameter is optional. If it is not set, the relevant field will maintain its default value.
• Required.Default
  • The parameter will be considered to be required only if the default value of the relevant field is null.

Generate a usage summary

CommanderParser<Options> parser = new CommanderParser<Options>();

Console.WriteLine(parser.Usage());

Will print out the following.

Usage: <exe> [options]
Options:
      -h, --help
        Print this message and exit.
        Default: False
      -i
        The ID
        Default: 42
    * -n, --name

An asterisk prefix implies that the option is required.

Exceptions

using Commander.NET.Exceptions;

try
{
	Options opts = CommanderParser.Parse<Options>(args);
}
catch (ParameterMissingException ex)
{
	// A required parameter was missing
	Console.WriteLine("Missing parameter: " + ex.ParameterName);
}
catch (ParameterFormatException ex)
{
	/*
	*	A string-parsing method raised a FormatException
	*	ex.ParameterName
	*	ex.Value
	*	ex.RequiredType
	*/
	Console.WriteLine(ex.Message);
}

Positional parameters

You can define positional parameters using the PositionalParameter attribute.

[PositionalParameter(0, "operation", Description = "The operation to perform.")]
public string Operation;

[PositionalParameter(1, "target", Description = "The host to connect to.")]
public string Host = "127.0.0.1";

When printing out the usage, positional parameters will be shown like the example below. However, they can be passed in any order in relation to the options.

Usage: Tests.exe [options] <operation> [target]
    operation: The operation to perform.
    target: The host to connect to.
Options:
    ...

Whether a parameter is required or not is defined exactly as shown above. This leaves room for error though, since for example the first positional parameter can be specified as optional and the second as required, which would leave you with a counter-intuitive interface design. The library does not currently enforce a good practice here - until a proper method of doing so is decided - so this is entirely up to the user.

In general, any argument passed that is neither the name of the parameter, nor the value of a non-boolean named parameter will be considered a positional parameter and assigned to the appropriate index.

You may also get all the positional parameters that were passed, using the `PositionalParameterList' attribute.

[PositionalParameterList]
public string[] Parameters;

Key-Value separators

By default, the parser will only consider key-value pairs that are separated by a space. You can change that by setting the Separators flags of the parser. The example below will parse both the --key value format and the --key=value format.

CommanderParser<Options> parser = new CommanderParser<Options>();
Options options = parser.Add(args)
                        .Separators(Separators.Space | Separators.Equals)
                        .Parse();

Currently available separators:

• Separators.Space
• Separators.Equals
• Separators.Colon
• Separators.All

Value validation

This section lists the ways with which you can validate, format or even convert the values that are passed to your parameters. Each individual value, goes through the following steps in that order:

1. Regex validation
2. Method validation
3. Formatting

Regular Expression validation

You can validate the values of a parameter through a regular expression, by setting the Regex property.

[Parameter("-n", "--name", Regex = "^john|mary$")]
public string Name;

If the regex match failes, a ParameterMatchException is thrown, as it shown below.

string[] args = { "-n", "james" };

try
{
	Options opts = CommanderParser.Parse<Options>(args);
	Console.WriteLine(opts.ToString());
}
catch (ParameterMatchException ex)
{
	// Parameter -n: value "james" did not match the regular expression "^john|mary$"
	Console.WriteLine(ex.Message);
}

Method validation

You can also use your own validation methods for values that are passed to a specific parameter by implementing the IParameterValidator interface.

If the Validate() method returns false, then a ParameterValidationException is thrown by the parser. Alternatively, you can use this method to throw your own exceptions, and they will rise to where you called you called the parser from.

The following basic example makes sure that the argument passed to the Age parameter is a positive integer.

using Commander.NET.Interfaces;

class PositiveInteger : IParameterValidator
{
	bool IParameterValidator.Validate(string name, string value)
	{
		int intVal;
		return int.TryParse(value, out intVal) && intVal > 0;
	}
}

[Parameter("-a", "--age", ValidateWith = typeof(PositiveInteger))]
public int Age;

Value formatting

Similar to method validation, you may want to declare your own methods for formatting - or event converting - certain parameter values. You may do this by implementing the IParameterFormatter interface.

The object returned by the Format() method will be directly set to the parameter with no other formatting or type conversion.

The following basic example, converts whatever value is passed to the --ascii parameter into a byte array.

using Commander.NET.Interfaces;

class StringToBytes : IParameterFormatter
{
	object IParameterFormatter.Format(string name, string value)
	{
		return Encoding.ASCII.GetBytes(value);
	}
}

[Parameter("--ascii", FormatWith = typeof(StringToBytes))]
public byte[] ascii;

Commands

Commands are usually a very important method for separating the different functionalities of a CLI program. Consider the following example, based on the popular git tool.

class Git
{
	[Command("commit")]
	public Commit Commit;

	[Command("push")]
	public Push Push;
}

class Commit
{
	[Parameter("-m")]
	public string Message;
}

class Push
{
	[PositionalParameter(0, "remote")]
	public string Remote;

	[PositionalParameter(1, "branch")]
	public string Branch;
}

Git git = new CommanderParser<Git>()
                         .Parse(args);

if (git.Commit != null)
{
	Console.WriteLine("Commiting: " + git.Commit.Message);
}
else if (git.Push != null)
{
	Console.WriteLine("Pushing to: " + git.Push.Remote + " " + git.Push.Branch);
}

• If at least one command is specified, but no command name is found in the arguments, the parser will raise a CommandMissingException
• Any arguments passed after the name of the command, are parsed and serialized into that command.

Command handlers

In the example above, to find out which command was issued we were comparing each command variable with null to find out which had been instantiated. Alternatively though, you can avoid any manual checks by using custom handlers for commands.

When the parsing and serialization of the arguments of an object has completed, the type of the command variable is checked for the ICommand interface. If the command type implements said interface, then the ICommand.Execute() is called on that object.

The object parent argument that is passed to this method, is the - already initialized - object, through which this command was called. In our example, this argument will contain the Git object.

class Commit : ICommand
{
	[Parameter("-m")]
	public string Message;

	void ICommand.Execute(object parent)
	{
		// The "commit" command was issued
	}
}

After the above callback, the base class is then checked for methods with the CommandHandler attribute. If any such method in the class has exactly one parameter and that parameter is of the same type as the command that is being executed, the method will be invoked.

class Git
{
	[Command("commit")]
	public Commit Commit;

	[Command("push")]
	public Push Push;

	[CommandHandler]
	public void PushCommand(Push push)
	{
		// The "push" command was issued
	}

	[CommandHandler]
	public void CommitCommand(Commit commit)
	{
		// The "commit" command was issued
	}
}

Private fields

By default, private and static fields, properties and methods are visible to the parser, even if their respective accessors are not public. You can change this behavior by manually passing the System.Reflection.BindingFlags flags to the parser.

CommanderParser<Options> parser = new CommanderParser<Options>();

parser.Bindings(BindingFlags.Public | BindingFlags.Instance);

//TODO

• Reverse positional indexing
• Specifying possible values. (f.e bacon|onions|tomatoes) Will be doable by default with regex, but enum support will be nice.
• Value type validation within the library with custom errors
• Recursive help flag for commands
• Support for appending the same parameter into collections
• Better support for mixing command parameters with parent parameters