Solution for overloaded operator constraint in .NET generics

Question

What would I do if I want to have a generic method that only accepts types that have overloaded an operator, for instance the subtraction operator. I tried using an interface as a constraint but interfaces can't have operator overloading.

What is the best way to achieve this?

Dmso

Do you have an example of where you are trying to use this? I can't think of anywhere that would be useful? – Kherson

As you have found, it is simply not possible to define a static method on an interface, so you cannot use it as a constraint for your generic method. Here is a somewhat complex workaround: codeproject.com/KB/cs/genericnumerics.aspx If you are using .NET 3.5, this can also be accomplished via LINQ expression trees, as follows: rogeralsing.com/2008/02/27/… – Paleo

A generic "Sum" method would be a simple example. T Sum<T>(IEnumerable<T> sequence); // where T has '+' operator – Dmso

Note that in Roger's blog we discuss/contrast the two implementations (they are very similar) - with the conclusion that the MiscUtil code (linked previously) is more developed. But they use the same fundamental approach. – Bombycid

possible duplicate of Define a generic that implements the + operator – Jumble

Answer 1

There is no immediate answer; operators are static, and cannot be expressed in constraints - and the existing primatives don't implement any specific interface (contrast to IComparable[<T>] which can be used to emulate greater-than / less-than).

However; if you just want it to work, then in .NET 3.5 there are some options...

I have put together a library here that allows efficient and simple access to operators with generics - such as:

T result = Operator.Add(first, second); // implicit <T>; here

It can be downloaded as part of MiscUtil

Additionally, in C# 4.0, this becomes possible via dynamic:

static T Add<T>(T x, T y) {
    dynamic dx = x, dy = y;
    return dx + dy;
}

I also had (at one point) a .NET 2.0 version, but that is less tested. The other option is to create an interface such as

interface ICalc<T>
{
    T Add(T,T)() 
    T Subtract(T,T)()
} 

etc, but then you need to pass an ICalc<T>; through all the methods, which gets messy.

Bombycid

I like the ability to use dynamic in .NET 4.0, it sure makes things much easier. However, it's worth pointing out that there will be a performance implication on using it because it has to do more work at runtime. I'd be interested to know how much of an impact it has, it needs some benchmarking I think. – Terrie

Benchmark already done; try the code from here: social.msdn.microsoft.com/Forums/en-US/vs2010ctpvbcs/thread/… – Bombycid

I tried out your library (for .NET 3.5), and I have a question: why doesn't the following line work: MiscUtil.Operator.Add("A", "B");. In my understanding, it should return "AB". – Shawana

@Shawana - well, we could add that, but that isn't really an arithmetic operation. And strictly speaking it isn't really a defined operator - it is currently the compiler (not the type itself) that provides that meaning of + for strings... – Bombycid

Is there any point to using generics if it's going to be converted to dynamic? Why not just make the parameters and the return code dynamic as well? Is there an advantage to mixing generics and dynamic in this way? – Crossopterygian

Answer 2

I found that IL can actually handle this quite well. Ex.

ldarg.0
ldarg.1
add
ret

Compiled in a generic method, the code will run fine as long as a primitive type is specified. It may be possible to extend this to call operator functions on non-primitive types.

See here.

Edette

Answer 3

You can solve this problem by using a delegate instead of an interface constraint.

public class Example
{
    public static T Add<T>(T left, T right, Func<T, T, T> addFunc) =>
        addFunc(left, right);
}

Define a method that takes a delegate as a parameter, and use it follows.

var result = Example.Add(10, 20, (x, y) => x + y);

Bead

Answer 4

There is a piece of code stolen from the internats that I use a lot for this. It looks for or builds using IL basic arithmetic operators. It is all done within an Operation<T> generic class, and all you have to do is assign the required operation into a delegate. Like add = Operation<double>.Add.

It is used like this:

public struct MyPoint
{
    public readonly double x, y;
    public MyPoint(double x, double y) { this.x=x; this.y=y; }
    // User types must have defined operators
    public static MyPoint operator+(MyPoint a, MyPoint b)
    {
        return new MyPoint(a.x+b.x, a.y+b.y);
    }
}
class Program
{
    // Sample generic method using Operation<T>
    public static T DoubleIt<T>(T a)
    {
        Func<T, T, T> add=Operation<T>.Add;
        return add(a, a);
    }

    // Example of using generic math
    static void Main(string[] args)
    {
        var x=DoubleIt(1);              //add integers, x=2
        var y=DoubleIt(Math.PI);        //add doubles, y=6.2831853071795862
        MyPoint P=new MyPoint(x, y);
        var Q=DoubleIt(P);              //add user types, Q=(4.0,12.566370614359172)

        var s=DoubleIt("ABC");          //concatenate strings, s="ABCABC"
    }
}

Operation<T> Source code courtesy of paste bin: http://pastebin.com/nuqdeY8z

with attribution below:

/* Copyright (C) 2007  The Trustees of Indiana University
 *
 * Use, modification and distribution is subject to the Boost Software
 * License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
 * http://www.boost.org/LICENSE_1_0.txt)
 *  
 * Authors: Douglas Gregor
 *          Andrew Lumsdaine
 *          
 * Url:     http://www.osl.iu.edu/research/mpi.net/svn/
 *
 * This file provides the "Operations" class, which contains common
 * reduction operations such as addition and multiplication for any
 * type.
 *
 * This code was heavily influenced by Keith Farmer's
 *   Operator Overloading with Generics
 * at http://www.codeproject.com/csharp/genericoperators.asp
 *
 * All MPI related code removed by ja72. 
 */

Kopans

Oh, I only saw dynamic keyword solution. – Kopans