Constraints

Constraints run after parsing, before handler invocation. Execution order: defaults first (may promote EmptyOption to DefaultOption), then option-level constraints, then command-level constraints.

Option-Level Constraints

Applied to a single option’s value. MustAssign and Default are applied automatically when the option is defined — the rest you apply explicitly.

Constraint	Applied	Behavior
`MustAssign`	automatic	Argument is required. Added when no `defaultArg` is supplied.
`Default`	automatic	Falls back to a value when the argument is omitted. Added when a `defaultArg` is supplied (`bool` always defaults to `false`).
`AcceptedValues`	`AcceptedValues(…)`	Restricts the option argument to a set of allowed values.
`CustomArgumentConstraint`	`ApplyConstraint(…)`	User-supplied predicate: `Func<T, bool>`

// AcceptedValues
cmdDef.AddOption<string>(
    key: "format", 
    defaultArg: "D", 
    help: "GUID format specifier.", 
    ("-f", "--format")
);
cmdDef["format"].AcceptedValues("N", "D", "B", "P", "X");

// CustomArgumentConstraint
cmdDef.AddOption<int>(
    key: "count", 
    defaultArg: 1,
     help: "Number of items.", 
     ("-c", "--count")
);
cmdDef["count"].ApplyConstraint<int>(
    cnt => cnt > 0 && cnt <= 100, 
    "Allowed Range", 
    "1..100."
);

cmdDef.AddOption<double>(key: "hours", help: "Hours logged.", ("-h", "--hours"));
cmdDef["hours"].ApplyConstraint<double>(
    h => h > 0 && 2 * h == (int)(2 * h),
    "Valid Increment",
    "Must be a positive multiple of 0.5."
);

cmdDef.AddOption<string>(key: "path", help: "Target path.", ("-p", "--path"));
cmdDef["path"].ApplyConstraint<string>(
    Path.IsPathFullyQualified,
    "Fully Qualified Path",
    "Must be a valid fully qualified path."
);

cmdDef.AddOption<string>(key: "comment", help: "Entry comment.", ("-c", "--comment"));
cmdDef["comment"].ApplyConstraint<string>(
    s => !string.IsNullOrWhiteSpace(s),
    "Not Whitespace",
    "Must contain a non-whitespace value."
);

Command-Level Constraints

Can access the full set of parsed options:

Constraint	Behavior
`MustAssignOneOf`	At least one option from a named set must be present
`MutuallyExclusiveSet`	At most one option from a named set may be present
`CustomCommandConstraint`	Receives an `ICommand` allowing access to the full set of parsed options.

// MustAssignOneOf
cmdDef.AddOption<bool>(key: "json", help: "Output as JSON.", (null, "--json"));
cmdDef.AddOption<bool>(key: "xml", help: "Output as XML.", (null, "--xml"));
cmdDef.AddOption<bool>(key: "csv", help: "Output as CSV.", (null, "--csv"));
cmdDef.MustAssignOneOf("json", "xml", "csv");

// MutuallyExclusiveSet
cmdDef.AddOption<bool>(key: "json", help: "Output as JSON.", (null, "--json"));
cmdDef.AddOption<bool>(key: "xml", help: "Output as XML.", (null, "--xml"));
cmdDef.AddOption<bool>(key: "csv", help: "Output as CSV.", (null, "--csv"));
cmdDef.MutuallyExclusiveSet("json", "xml", "csv");
// Can be combined with MustAssignOneOf to enforce 1 and only 1.
cmdDef.MustAssignOneOf("json", "xml", "csv");

// CustomCommandConstraint
cmdDef.AddOption<int>(key: "age", help: "Applicant age.", ("-a", "--age"));
cmdDef.ApplyConstraint(
    cmd => cmd["age"].GetValue<int>() >= 18,
    "Age Restriction",
    "Must be 18 or older."
);

Option Types Handlers