C# Generic Operators

问题:

I am trying to implement a generic operator like so:

class Foo
{
   public static T operator +<T>(T a, T b) 
   {
       // Do something with a and b that makes sense for operator + here
   }
}

Really what I'm trying to do is gracefully handle inheritance. With a standard operator + in Foo, where T is instead "Foo", if anyone is derived from Foo (say Bar inherits Foo), then a Bar + Bar operation will still return a Foo. I was hoping to solve this with a generic operator +, but I just get a syntax error for the above (at the <) making me believe that such code is not legal.

Is there a way to make a generic operator?

回答1:

No, you can't declare generic operators in C#.

Operators and inheritance don't really mix well.

If you want Foo + Foo to return a Foo and Bar + Bar to return a Bar, you will need to define one operator on each class. But, since operators are static, you won't get the benefits of polymorphism because which operator to call will be decided at compile-time:

Foo x = new Bar();
Foo y = new Bar();
var z = x + y; // calls Foo.operator+;

回答2:

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

static T Add<T>(T a, T b) {
    //TODO: re-use delegate!
    // declare the parameters
    ParameterExpression paramA = Expression.Parameter(typeof(T), "a"),
        paramB = Expression.Parameter(typeof(T), "b");
    // add the parameters together
    BinaryExpression body = Expression.Add(paramA, paramB);
    // compile it
    Func<T, T, T> add = Expression.Lambda<Func<T, T, T>>(body, paramA, paramB).Compile();
    // call it
    return add(a,b);       
}

回答3:

You can just define operator in a generic class Foo.

You can also create real generic operators, but C# compiler won't use them.

[System.Runtime.CompilerServices.SpecialName]
public static T op_Addition<T>(T a, T b) { ... }

回答4:

You cannot declare generic operators in C# - I am not sure on the reasoning but assume it's a usefulness vs effort thing for the implementation team (I believe there might be a post on here with Jon Skeet discussing it, or perhaps on his blog when he discussed things he'd like to see in C#).

Indeed, you cannot even use operators with generics in C#.

This is because generics must be applicable for all possible types that could be provided. This is why you must scope the generic type to classes when you want to use == as below:

void IsEqual<T>(T x, T y) where T : class
{
    return x == y;
}

Unfortunately you cannot do:

void Add<T>(T x, T y)  where T : operator +
{
    return x + y;
}

You might also be interested in this short summary article I came across.

回答5:

Was searching for the same thing and google brought me here.... I wasn't too happy about the accepted answer and was looking for a workaround.

I managed to implement this using generics. Here is the Foo and Bar class:

    class Foo
    {
        private int value;

        public Foo(int x)
        {
            value = x;
        }
        public virtual int getVal()
        {
            return value;
        }
    }

    class Bar : Foo
    {
        private int derivedValue;

        public Bar(int x):base(x) 
        {
            derivedValue = x;
        }
        public override int getVal()
        {
            return derivedValue;
        }
    }

Then a generic class containing the operators but restricted to type of Foo and derived from Foo:

    class GenericOp<T> where T : Foo
    {
        private T value;

        public GenericOp(T x)
        {
            value = x;
        }
        public static Foo operator +(GenericOp<T> a, GenericOp<T> b) 
        {
            return new Foo(a.value.getVal() + b.value.getVal());
        }
    }

Some usage code showing you always get back a Foo as well as preventing you from mixing the types:

Foo f1 = new Foo(1);
Foo f2 = new Foo(2);
Bar b1 = new Bar(10);
Bar b2 = new Bar(20);

GenericOp<Foo> left = new GenericOp<Foo>(f1);
GenericOp<Foo> right = new GenericOp<Foo>(f2);
Foo res = left + right;

GenericOp<Bar> left1 = new GenericOp<Bar>(b1);
GenericOp<Bar> right1 = new GenericOp<Bar>(b2);
Foo res1 = left1 + right1;

GenericOp<Foo> left2 = new GenericOp<Foo>(f1);
GenericOp<Bar> right2 = new GenericOp<Bar>(b1);
//Foo res2 = left2 + right2; //this fails and rightfully so.