louthy/language-ext · error · ArgumentException

must be of type Velocity

Error message

must be of type Velocity

What it means

Velocity's explicit CompareTo(object?) throws ArgumentException('must be of type Velocity') when the argument is non-null but not a Velocity. The library follows the standard IComparable pattern: null yields 1, a real Velocity delegates to the typed overload, anything else is a caller bug. Only object/IComparable-typed dispatch paths can reach this throw.

Solutions

1. Only compare same-typed values; keep Velocity collections typed as List<Velocity>
2. Use the generic CompareTo(Velocity) overload directly
3. Type-check with `obj is Velocity` before the non-generic call
4. Catch ArgumentException when consuming untyped external data

Example fix

// before
if (((IComparable)v).CompareTo(new TimeSq(1)) > 0) { } // ArgumentException
// after
if (other is Velocity v2)
{
    if (v.CompareTo(v2) > 0) { }
}

if (obj is not Velocity) return -1; // or throw, per policy — before calling CompareTo(object)

static bool IsVelocity(object? o) => o is Velocity;

try { r = ((IComparable)vel).CompareTo(obj); }
catch (ArgumentException) { r = string.Compare(vel.GetType().Name, obj?.GetType().Name); }

When it happens

Trigger: Comparing a Velocity against a different unit type (Time, VelocitySq, boxed primitives) via the non-generic CompareTo or through non-generic sort/search APIs (ArrayList.BinarySearch, Array.Sort(object[])).

Common situations: Mixing unit structs in untyped collections; reflection-based comparers; code migrated from object-based collections to unit types without tightening types.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• must be of type Time
• must be of type TimeSq
• must be of type VelocitySq
• The minimum value for
• Ord attribute should have a struct type that derives from…

AI-assisted analysis of louthy/language-ext@2f0e362824 (2026-09-15). Data as JSON: /api/errors/8d681f5d9793e29a. Report an issue: GitHub.

Appendix: source

Thrown at LanguageExt.Core/Units of Measure/Velocity.cs:45

    public bool Equals(Velocity other) =>
        Value.Equals(other.Value);

    public bool Equals(Velocity other, double epsilon) =>
        Math.Abs(other.Value - Value) < epsilon;

    public override bool Equals(object? obj) =>
        obj is Velocity velocity && Equals(velocity);

    public override int GetHashCode() =>
        Value.GetHashCode();

    public int CompareTo(object? obj) =>
        obj switch
        {
            null           => 1,
            Velocity other => CompareTo(other),
            _              => throw new ArgumentException($"must be of type {nameof(Velocity)}")
        };

    public int CompareTo(Velocity other) =>
        Value.CompareTo(other.Value);

    public Velocity Add(Velocity rhs) =>
        new (Value + rhs.Value);

    public Velocity Subtract(Velocity rhs) =>
        new (Value - rhs.Value);

    public Velocity Multiply(double rhs) =>
        new (Value * rhs);

    public Velocity Divide(double rhs) =>
        new (Value / rhs);

    public static Velocity operator *(Velocity lhs, double rhs) =>

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