This is something that has been on many people's wish lists for a while, but now with the addition of generic math it has gained a new use case.

Lets say I want to define a generic 3d vector interface:

public interface IVector3<T>
{
    T X { get; set; }
    T Y { get; set; }
    T Z { get; set; }
}

As well as a float implementation for this example:

public struct Vector3f : IVector3<float>
{
    public float X { get; set; }
    public float Y { get; set; }
    public float Z { get; set; }
}

I then want to define a cross product method that works for any vector, as long as the numeric constraints are met:

public static class Vector
{
    public static TVector Cross<TVector, TElement>(TVector v1, TVector v2)
        where TVector : IVector3<TElement>, new()
        where TElement :
            System.Numerics.ISubtractionOperators<TElement, TElement, TElement>,
            System.Numerics.IMultiplyOperators<TElement, TElement, TElement>
    {
        var result = new TVector();
        result.X = v1.Y * v2.Z - v1.Z * v2.Y;
        result.Y = v1.Z * v2.X - v1.X * v2.Z;
        result.Z = v1.X * v2.Y - v1.Y * v2.X;
        return result;
    }
}

Perfect! Lets try it out:

var v1 = new Vector3f();
var v2 = new Vector3f();
var v3 = Vector.Cross(v1, v2);

So close:

error CS0411: The type arguments for method 'Vector.Cross<TVector, TElement>(TVector, TVector)' cannot be inferred from the usage. Try specifying the type arguments explicitly.

So instead you have to write the following:

var v1 = new Vector3f();
var v2 = new Vector3f();
var v3 = Vector.Cross<Vector3f, float>(v1, v2);

Which is not something we want to type all the time. We want C# to infer the types for us.

An alternative would be to change the method signature to Cross(IVector3<TElement> v1, IVector3<TElement> v2). This however would lead to boxing, which has significant performance implications.

So I suggest the language team to reconsider constaint type inference. Right now generic math is unable to reach its full potential without it. Vectors and Matrices, Complex numbers and Quaternions, Points, Sizes and Rectangles, these are all multi value math structures that can greatly benefit from this feature.

Uh oh!

Constraint type inference #6930

Uh oh!

Uh oh!

rick-de-water Jan 30, 2023

rick-de-water
Jan 30, 2023