Units of measure in C# - almost

Question

Inspired by Units of Measure in F#, and despite asserting (here) that you couldn\'t do it in C#, I had an idea the other day which I\'ve been playing around with.

Answer 1

You are missing dimensional analysis. For example (from the answer you linked to), in F# you can do this:

let g = 9.8

and it will generate a new unit of acceleration, derived from meters and seconds (you can actually do the same thing in C++ using templates).

In C#, it is possible to do dimensional analysis at runtime, but it adds overhead and doesn't give you the benefit of compile-time checking. As far as I know there's no way to do full compile-time units in C#.

Whether it's worth doing depends on the application of course, but for many scientific applications, it's definitely a good idea. I don't know of any existing libraries for .NET, but they probably exist.

If you are interested in how to do it at runtime, the idea is that each value has a scalar value and integers representing the power of each basic unit.

class Unit
{
    double scalar;
    int kg;
    int m;
    int s;
    // ... for each basic unit

    public Unit(double scalar, int kg, int m, int s)
    {
       this.scalar = scalar;
       this.kg = kg;
       this.m = m;
       this.s = s;
       ...
    }

    // For addition/subtraction, exponents must match
    public static Unit operator +(Unit first, Unit second)
    {
        if (UnitsAreCompatible(first, second))
        {
            return new Unit(
                first.scalar + second.scalar,
                first.kg,
                first.m,
                first.s,
                ...
            );
        }
        else
        {
            throw new Exception("Units must match for addition");
        }
    }

    // For multiplication/division, add/subtract the exponents
    public static Unit operator *(Unit first, Unit second)
    {
        return new Unit(
            first.scalar * second.scalar,
            first.kg + second.kg,
            first.m + second.m,
            first.s + second.s,
            ...
        );
    }

    public static bool UnitsAreCompatible(Unit first, Unit second)
    {
        return
            first.kg == second.kg &&
            first.m == second.m &&
            first.s == second.s
            ...;
    }
}

If you don't allow the user to change the value of the units (a good idea anyways), you could add subclasses for common units:

class Speed : Unit
{
    public Speed(double x) : base(x, 0, 1, -1, ...); // m/s => m^1 * s^-1
    {
    }
}

class Acceleration : Unit
{
    public Acceleration(double x) : base(x, 0, 1, -2, ...); // m/s^2 => m^1 * s^-2
    {
    }
}

You could also define more specific operators on the derived types to avoid checking for compatible units on common types.