An OptionSet is the definition of all possible commandline options.

new OptionSet("-", "--")
	.Add(new[] { "c" }, new[] { "command" })
	.Add(new[] { "h" }, new[] { "help" })

- is the prefix and -- is the alternative prefix. In this example, following options are configured:

• -c
• --command
• -h
• --help

Each option can have multiple parameters. Example:

-f file1.txt file2.txt -o output.txt

After the OptionSet.Parse method comes the handling of user provided commandline arguments.

The handler executes the Action<string[]> with the arguments after the commandline, i.e.

CommandLine: -c arg1 arg2
Output: -c executed with arg1 arg2

new OptionSet("-", "--")
	.Add(new[] { "c" }, new[] { "command" })
	.Add(new[] { "h" }, new[] { "help" })
	.Parse(args) // From Main() method
	.Handle("c", arguments =>
	{
		Console.WriteLine("-c executed with " + string.Join(" ", arguments));
	});

Validate provides a callback that is invoked, if a specific validation rule is not met.

If none of the existing validation rules satisfy your requirements, use Validate.Custom to implement a custom validation rule.

new OptionSet("-", "--")
	.Add(new[] { "c" }, new[] { "command" })
	.Add(new[] { "h" }, new[] { "help" })
	.Parse(args)
	.Validate.OptionRequired("c", () =>
	{
		Console.WriteLine("-c is mandatory");
	})
	.Handle(
		// ...
	);

Assert works similar to Validate, however instead of callbacks, a CommandLineParserException is thrown.

Use assertion, if an exception is sufficient and manual error handling is not required.

new OptionSet("-", "--")
	.Add(new[] { "c" }, new[] { "command" })
	.Add(new[] { "h" }, new[] { "help" })
	.Parse(args)
	.Assert.OptionRequired("c")
	.Handle(
		// ...
	);