C# 4.0 dynamic: A potential performant solution to numeric generics?

Question

After coming up against this problem myself in trying to implement a generic Vector2<int/float/double> in C#, I've done a bunch of investigation into this problem, also described in this question:

Less generic generics? A possible solution for arithmetic in C# generics

These links contain some more background information and fascinating solution approaches:

https://jonskeet.uk/csharp/miscutil/usage/genericoperators.html

http://www.codeproject.com/KB/cs/genericnumerics.aspx

Now that C# 4.0 is out with its new versatile dynamic type, my question for the brilliant SO community, is this: is it a tool that could be used perhaps to build performant, generic Vector/Matrix/etc. numeric types?

Clearly a Vector2 could be built by simply like so:

public struct Vector2
{
    public dynamic X;
    public dynamic Y;

    public Vector2(dynamic x, dynamic y)
    {
        this.X = x;
        this.Y = y;
    }

    public static Vector2 operator+(Vector2 a, Vector2 b)
    {
        return new Vector2(a.X + b.X, a.Y + b.Y);
    }
}

but with this approach we have no type constraint here, so you could make a Vector2(3, 12.4572). Is there a way that we could mix dynamic members with a type parameter Vector2<int> to perform our math operations as would be done with ints?

Perhaps some form of casting could be used to ensure this.X is a T, though I don't know how that would perform.

Answer 1

Only you can tell if dynamic operator invocations will meet your performance requirements, but it's certainly possible to address some of your type-safety concerns with generics - there's no reason that everything has to be checked at run-time just because of one little dynamic call:

// Consider making this type immutable
public struct Vector2<T>
{
    public T X;
    public T Y;

    public Vector2(T x, T y)
    {
        this.X = x;
        this.Y = y;
    }

    //  The only dangerous operation on the type
    public static Vector2<T> operator +(Vector2<T> a, Vector2<T> b)
    {
        return new Vector2<T>((dynamic)a.X + b.X, (dynamic)a.Y + b.Y);
    }
}

Now, the only dangerous operation is to actually add 2 vectors of the same type (the addition operator needs to work as expected on the type-argument), but everything else is perfectly type-safe, as it should be. You can't do new Vector<int>("a", 5), add a Vector<int> and a Vector<string>, or assign the addition of two Vector<int>s to a Vector<string>. Note that none of these errors would have been caught at compile-time with your original solution.

Note that:

1. There's nothing to stop you from using generics here but going down the compiling-an-expression-tree route for the addition instead of dynamic. Delegate invocations aren't free, but they should in theory be faster than the dynamic approach in this case - at the very least, you avoid boxing value-types. Only you can tell if they will be fast enough, though.

2. In all cases, consider writing a static-constructor that validates that the type-argument in fact has a suitable addition operator, so that type-errors happen early on in the game.

---

EDIT (OP isn't satisfied with the performance of dynamic here):

The expression-tree approach would look something like:

public struct Vector2<T>
{
    private static readonly Func<T, T, T> Add;

    // Create and cache adder delegate in the static constructor.
    // Will throw a TypeInitializationException
    // if you can't add Ts or if T + T != T 
    static Vector2()
    {
        var firstOperand = Expression.Parameter(typeof(T), "x");
        var secondOperand = Expression.Parameter(typeof(T), "y");
        var body = Expression.Add(firstOperand, secondOperand);
        Add = Expression.Lambda<Func<T, T, T>>
              (body, firstOperand, secondOperand).Compile();
    }

    public T X;
    public T Y;

    public Vector2(T x, T y)
    {
        this.X = x;
        this.Y = y;
    }

    public static Vector2<T> operator +(Vector2<T> a, Vector2<T> b)
    {
        // Delegate invocation instead of dynamic operator invocation.
        return new Vector2<T>(Add(a.X, b.X), Add(a.Y, b.Y));
    }
}