Recommended way to create 3 dimensional magnetic field?

[krwq]

We're currently considering to use this library in https://github.com/dotnet/iot/ . We have initially started developing very similar APIs and then noticed this library so we decided to give it a try since it already solves many problems we were about to hit.

We have converted our Temperature/Pressure structs to this library without any issues (see dotnet/iot#1052) but now we're trying to use 3-dimensional magnetic field which is the output of IMU (i.e. BNO055 sensor). We haven't ourselves developed such unit yet and used Vector3.

What's the recommended way to achieve 3-dimensional magnetic field?

[pgrawehr]

Do I understand correctly that you would like to have a Vector3<MagneticField> or Vector3<Speed> instance? I guess that will end up with the same problems that were also discussed in #695.

Consider this code:

public class Vector3<T>
{
    private T _value;
    public Vector3(T value)
    {
        _value = value;
    }
    
    public static Vector3<T> operator+(Vector3<T> a, Vector3<T> b)
    {
        return new Vector3(a._value + b._value); // ERROR: T does not have an operator +
    }
    // ....
}

Unfortunatelly the above doesn't work, because the generic type "T" (aka any type) does not have a defined + operator. There's no way to add a type constraint on "something that is a number". And declaring interfaces (IImplementsAddition) for operator methods is also not possible, because in C# they need to be static by definition 😢 ...

This works:

    public static Vector3<T> operator+(Vector3<T> a, Vector3<T> b)
    {
        dynamic a1 = a;
        dynamic b1 = b;
        return new Vector3(a1._value + b1._value); // Works!
    }

But of course, this comes with a performance penalty (I don't know how big it is) and the downside that problems will pop up only at runtime and not at compile time.

[angularsen]

Hi,

UnitsNet does not currently support XYZ-dimensional quantities in a generic way.
Some discussion in #695 on why this was not straight forward to add, but it is absolutely possible if someone wants to champion it and a performant proposal is outlined in #666.

You can, however, easily add your own wrapper types for specific quantities.
Something like this:

public struct MagneticField3
{
    public MagneticField X { get; }
    public MagneticField Y { get; }
    public MagneticField Z { get; }
    
    public MagneticField3(MagneticField x, MagneticField y, MagneticField z) { ... }

    // Operator overloads for arithmetic
    public static MagneticField3 operator +(MagneticField3 left, MagneticField3 right) { ... }
    public static MagneticField3 operator -(MagneticField3 left, MagneticField3 right) { ... }
    public static MagneticField3 operator *(double left, MagneticField3 right) { ... }
    public static MagneticField3 operator /(MagneticField3 left, double right) { ... }

Did that answer your question?

[krwq]

@pgrawehr not necessarily, you could alternatively have MagneticField3 struct. Generally speaking the magnetic field definition talks about vectors and not scalars.

I agree it's not ideal that above cannot be expressed too well in C# and generic vector would be a perfect solution.

[tmilnthorp]

We could use the code waiting in #698 to do this in a generic way, and without a large performance hit.

[pgrawehr]

Sounds good, but would apparently also need the definition of the Vector and Matrix classes to be useful, right? A matrix multiplication would be a good test case for the performance impact.

[krwq]

Honestly for units I only care about vector. Matrices are probably only used in more advanced physics (i.e. jacobian) which usually need super high performance in which case you'd likely write your own version anyway. Good idea for completeness but I'd personally skip it for now.

@angularsen that should be fine for now although this is something I'd really love to see here in the future (although understand the limitations and why not yet)

[tmilnthorp]

I created #801 as a POC. I assume you would expect to write something as follows:

            var length1X = Length.FromMeters(1.0);
            var length1Y = Length.FromMeters(2.0);
            var length1Z = Length.FromMeters(3.0);
            var length2X = Length.FromMeters(4.0);
            var length2Y = Length.FromMeters(5.0);
            var length2Z = Length.FromMeters(6.0);

            var vector1 = new Vector3<Length>(length1X, length1Y, length1Z);
            var vector2 = new Vector3<Length>(length2X, length2Y, length2Z);

            var result = vector1 + vector2;

            var expectedX = Length.FromMeters(5.0);
            var expectedY = Length.FromMeters(7.0);
            var expectedZ = Length.FromMeters(9.0);

            Assert.Equal( new Vector3<Length>( expectedX, expectedY, expectedZ ), result );

[pgrawehr]

Nice start, and nice that this works with base types, too:

        [Fact]
        public void WorksWithBaseTypes()
        {
            double length1X = 1.0;
            double length1Y = 2.0;
            double length1Z = 3.0;
            double length2X = 4.0;
            double length2Y = 5.0;
            double length2Z = 6.0;

            var vector1 = new Vector3<double>(length1X, length1Y, length1Z);
            var vector2 = new Vector3<double>(length2X, length2Y, length2Z);

            var result = vector1 + vector2;

            double expectedX = 5.0;
            double expectedY = 7.0;
            double expectedZ = 9.0;

            Assert.Equal(new Vector3<double>(expectedX, expectedY, expectedZ), result);
        }

[pgrawehr]

This doesn't make a lot of sense, but I just tried to do:

        [Fact]
        public void ThisIsWeird()
        {
            var vector1 = new Vector3<String>("A", "B", "C");
            var vector2 = new Vector3<String>("D", "E", "F");

            Assert.Equal("AD", (vector1 + vector2).X);
        }

Not that I would have a meaningful usage for this, but this failed with

System.TypeInitializationException : The type initializer for 'AddImplementation`3' threw an exception.
---- System.InvalidOperationException : The binary operator Add is not defined for the types 'System.String' and 'System.String'.
   at UnitsNet.CompiledLambdas.AddImplementation`3.Invoke(TLeft left, TRight right) in C:\projects\UnitsNet\UnitsNet\CompiledLambdas.cs:line 224
   at UnitsNet.CompiledLambdas.Add[T](T left, T right) in C:\projects\UnitsNet\UnitsNet\CompiledLambdas.cs:line 61
   at UnitsNet.Vector3`1.op_Addition(Vector3`1 left, Vector3`1 right) in C:\projects\UnitsNet\UnitsNet\Vector3.cs:line 59
   at UnitsNet.Tests.VectorTests.ThisIsWeird() in C:\projects\UnitsNet\UnitsNet.Tests\VectorTests.cs:line 73

String does have a binary + operator, so this message confuses me a bit and I'm worried we're overlooking some meaningful use case here?

[tmilnthorp]

String does not have a + operator 😃

It is syntactic sugar for the compiler to generate string.Concat.