为什么不Nullable <T> 实现IComparable？

Question

The answer to this is probably glaringly obvious, but I'll stoop to asking anyway.

I was in the midst of writing a Range<T> class, and in the process realized I needed a comparer for it. So, I wrote a generic comparer, derived, naturally, from Comparer<T> :

public class Range<T> where T : IComparable
{

    // Simplified here for brevity -- actually uses the comparer
    public T Min { get; set; }

    // Simplified here for brevity -- actually uses the comparer
    public T Max { get; set; }

    private class DefaultComparer : Comparer<T> 
    {
        public override int Compare(T x, T y)
        {
            return x == null && y == null 
                ? 0 
                : x == null 
                    ? -1 
                    : y == null 
                        ? 1 
                        : x.CompareTo(y);
        }
    }
}

Now, this all works fine and well, until you pass a Nullable<T> as the type. For example:

public Range<DateTime?> DateSentFilter { get; set; }

This breaks horribly, because, of course, Nullable<T> doesn't implement IComparable . And this got me to thinking: Why not?

Given how a comparer is generally written, why couldn't it?

Does anyone with deep knowledge of this stuff have any idea? Is there a scenario that would prohibit this from being done?

Answer 1

Well, the generic type provided to Nullable<T> doesn't need to implement IComparable itself.

You have provided this condition yourself: T : IComparable , but that is not necessarily true for the T in Nullable<T> . I will agree that Nullable<T> is often used on primitive types that do implement IComparable but you could use it on structs, and they don't have IComparable implemented.

Answer 2

This is a comment from the source code :

// Warning, don't put System.Runtime.Serialization.On*Serializ*Attribute
// on this class without first fixing ObjectClone::InvokeVtsCallbacks
// Also, because we have special type system support that says a a boxed Nullable<T>
// can be used where a boxed<T> is use, Nullable<T> can not implement any intefaces
// at all (since T may not).   Do NOT add any interfaces to Nullable!
// 

As it says, Nullable<T> can't implement any interfaces for given reasons.

Now, the second question is: if it could, would they implement IComparable ? No. Patrick has told why. Not every T implements IComparable , it just wouldn't make sense.