Covariance and Contravariance in C#

Question

I will start by saying that I am Java developer learning to program in C#. As such I do comparisons of what I know with what I am learning.

I have been playing with C# generics for a few hours now, and I have been able to reproduce the same things I know in Java in C#, with the exception of a couple of examples using covariance and contravariance. The book I am reading is not very good in the subject. I will certainly seek more info on the web, but while I do that, perhaps you can help me find a C# implementation for the following Java code.

An example is worth a thousand words, and I was hoping that by looking a good code sample I will be able to assimilate this more rapidly.

Covariance

In Java I can do something like this:

public static double sum(List<? extends Number> numbers) {
    double summation = 0.0;
    for(Number number : numbers){
        summation += number.doubleValue();
    }
    return summation;
}

I can use this code as follows:

List<Integer> myInts = asList(1,2,3,4,5);
List<Double> myDoubles = asList(3.14, 5.5, 78.9);
List<Long> myLongs = asList(1L, 2L, 3L);

double result = 0.0;
result = sum(myInts);
result = sum(myDoubles)
result = sum(myLongs);

Now I did discover that C# supports covariance/contravariance only on interfaces and as long as they have been explicitly declared to do so (out/in). I think I was not able to reproduce this case, because I could not find a common ancestor of all numbers, but I believe that I could have used IEnumerable to implement such thing if a common ancestor exists. Since IEnumerable is a covariant type. Right?

Any thoughts on how to implement the list above? Just point me into the right direction. Is there any common ancestor of all numeric types?

Contravariance

The contravariance example I tried was the following. In Java I can do this to copy one list into another.

public static void copy(List<? extends Number> source, List<? super Number> destiny){
    for(Number number : source) {
       destiny.add(number);
    }
}

Then I could use it with contravariant types as follows:

List<Object> anything = new ArrayList<Object>();
List<Integer> myInts = asList(1,2,3,4,5);
copy(myInts, anything);

My basic problem, trying to implement this in C# is that I could not find an interface that was both covariant and contravariant at the same time, as it is case of List in my example above. Maybe it can be done with two different interfaces in C#.

Any thoughts on how to implement this?

Thank you very much to everyone for any answers you can contribute. I am pretty sure I will learn a lot from any example you can provide.

Answer 1

Rather than answering your questions directly I'm going to answer some slightly different questions:

Does C# have a way to genericize over types that support arithmetic operators?

Not easily, no. It would be nice to have the capability of making a Sum<T> method that could add integers, doubles, matrices, complex numbers, quaternions... and so on. Though this is a fairly frequently requested feature, it is also a big feature and it has never been high enough on the priority list to justify its inclusion in the language. I would personally like it, but you should not expect it in C# 5. Perhaps in a hypothetical future version of the language.

What is the difference between Java's "call site" covariance/contravariance and C#'s "declaration site" covariance/contravariance?

The fundamental difference at the implementation level is of course that as a practical matter, Java generics are implemented via erasure; though you get the benefits of a pleasant syntax for generic types, and compile-time type checking, you don't necessarily get the performance benefits or runtime type system integration benefits that you would in C#.

But that's really more of an implementation detail. The more interesting difference from my perspective is that Java's variance rules are enforced locally and C#'s variance rules are enforced globally.

That is to say: certain variant conversions are dangerous because they imply that certain not-type-safe operations will not be caught by the compiler. The classic example is:

• A tiger is a mammal.
• A list of X is covariant in X. (Suppose.)
• A list of tigers is therefore a list of mammals.
• A list of mammals can have a giraffe inserted.
• Therefore you can insert a giraffe into a list of tigers.

Which clearly violates type safety, as well as the safety of the giraffe.

C# and Java use two different techniques to prevent this type safety violation. C# says that when the I<T> interface is declared, if it is declared as covariant then there must be no method of the interface which takes in a T. If there is no method for inserting a T into a list, then you will never insert a giraffe into a list of tigers because there is no method for inserting anything.

Java by contrast says that at this local site, we get to treat the type covariantly and we promise not to call any methods right here that might violate type safety.

I do not have enough experience with Java's feature to say which is "better" under what circumstances. The Java technique is certainly interesting.

Answer 2

For the 2nd part to your question, you don't need contravariance, all you need to do is state that the first type can be cast to the second. Again, use the where TSource: TDest syntax to do this. Here is a full example (that shows how to do it with an extension method):

static class ListCopy
{
    public static void ListCopyToEnd<TSource, TDest>(this IList<TSource> sourceList, IList<TDest> destList)
        where TSource : TDest // This lets us cast from TSource to TDest in the method.
    {
        foreach (TSource item in sourceList)
        {
            destList.Add(item);
        }
    }
}

class Program
{
    static void Main(string[] args)
    {
        List<int> intList = new List<int> { 1, 2, 3 };
        List<object> objList = new List<object>(); ;

        ListCopy.ListCopyToEnd(intList, objList);
        // ListCopyToEnd is an extension method
        // This calls it for a second time on the same objList (copying values again).
        intList.ListCopyToEnd(objList);

        foreach (object obj in objList)
        {
            Console.WriteLine(obj);
        }
        Console.ReadLine();
    }