Is there a constraint that restricts my generic me

Question

Can anyone tell me if there is a way with generics to limit a generic type argument T to only:

• Int16
• Int32
• Int64
• UInt16
• UInt32
• UInt64

I'm aware of the where keyword, but can't find an interface for only these types,

Something like:

static bool IntegerFunction<T>(T value) where T : INumeric 

Answer 1

There is no way to restrict templates to types, but you can define different actions based on the type. As part of a generic numeric package, I needed a generic class to add two values.

    class Something<TCell>
    {
        internal static TCell Sum(TCell first, TCell second)
        {
            if (typeof(TCell) == typeof(int))
                return (TCell)((object)(((int)((object)first)) + ((int)((object)second))));

            if (typeof(TCell) == typeof(double))
                return (TCell)((object)(((double)((object)first)) + ((double)((object)second))));

            return second;
        }
    }

Note that the typeofs are evaluated at compile time, so the if statements would be removed by the compiler. The compiler also removes spurious casts. So Something would resolve in the compiler to

        internal static int Sum(int first, int second)
        {
            return first + second;
        }

Answer 2

There is no single interface or base class that they all inherit (that is not also inherited by other classes) so the simple answer is no.

I do wonder why this is an issue though. What are you wanting to do inside your IntegerFunction class that can only be done to integers?

Answer 3

Hejlsberg has described the reasons for not implementing the feature in an interview with Bruce Eckel.

I have to admit, though, that I don't know how he thinks his proposed workaround will work. His proposal is to defer arithmetic operations to some other generic class (read the interview!). How does this help? IMHO, not much.

Answer 4

Workaround using policies:

interface INumericPolicy<T>
{
    T Zero();
    T Add(T a, T b);
    // add more functions here, such as multiplication etc.
}

struct NumericPolicies:
    INumericPolicy<int>,
    INumericPolicy<long>
    // add more INumericPolicy<> for different numeric types.
{
    int INumericPolicy<int>.Zero() { return 0; }
    long INumericPolicy<long>.Zero() { return 0; }
    int INumericPolicy<int>.Add(int a, int b) { return a + b; }
    long INumericPolicy<long>.Add(long a, long b) { return a + b; }
    // implement all functions from INumericPolicy<> interfaces.

    public static NumericPolicies Instance = new NumericPolicies();
}

Algorithms:

static class Algorithms
{
    public static T Sum<P, T>(this P p, params T[] a)
        where P: INumericPolicy<T>
    {
        var r = p.Zero();
        foreach(var i in a)
        {
            r = p.Add(r, i);
        }
        return r;
    }

}

Usage:

int i = NumericPolicies.Instance.Sum(1, 2, 3, 4, 5);
long l = NumericPolicies.Instance.Sum(1L, 2, 3, 4, 5);
NumericPolicies.Instance.Sum("www", "") // compile-time error.

The solution is compile-time safe. CityLizard Framework provides compiled version for .NET 4.0. The file is lib/NETFramework4.0/CityLizard.Policy.dll.

It's also available in Nuget: https://www.nuget.org/packages/CityLizard/. See CityLizard.Policy.I structure.

Answer 5

If all you want is use one numeric type, you could consider creating something similar to an alias in C++ with using.

So instead of having the very generic

T ComputeSomething<T>(T value1, T value2) where T : INumeric { ... }

you could have

using MyNumType = System.Double;
T ComputeSomething<MyNumType>(MyNumType value1, MyNumType value2) { ... }

That might allow you to easily go from double to int or others if needed, but you wouldn't be able to use ComputeSomething with double and int in the same program.

But why not replace all double to int then? Because your method may want to use a double whether the input is double or int. The alias allows you to know exactly which variable uses the dynamic type.

Answer 6

There's no constraint for this. It's a real issue for anyone wanting to use generics for numeric calculations.

I'd go further and say we need

static bool GenericFunction<T>(T value) 
    where T : operators( +, -, /, * )

Or even

static bool GenericFunction<T>(T value) 
    where T : Add, Subtract

Unfortunately you only have interfaces, base classes and the keywords struct (must be value-type), class (must be reference type) and new() (must have default constructor)

You could wrap the number in something else (similar to INullable<T>) like here on codeproject.

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You could apply the restriction at runtime (by reflecting for the operators or checking for types) but that does lose the advantage of having the generic in the first place.