How to implement generic method in C# having multiple constraints

Question

I want to write a generic method:

void Foo<T>(IList<T> list)

In the method, I want to compare elements of list either by using operator < if T is primitive type or Compare() if T implements IComparable.

Is this possible? If not, it means C# generics is very limited in use I think.

What I mean is: in C++, it is up to compiler to check if a call is valid at instantiation. But C# has stronger requirement and hence narrow use of generics. — Sheen
– Sheen, Commented Jan 15, 2014 at 14:00
You say "narrow" I say "absolutely fine in the vast majority of cases where I've wanted to use generics." — Jon Skeet
– Jon Skeet, Commented Jan 15, 2014 at 14:07
@Jon Skeet, really "absolutely fine", even when you need so tricky way to call operator+ in a generic method? (Yes, I read your other article just now). IMO, C# at least needs some contract specification for generics that say "T requires to have operator+ overloaded" or "T requires to have method of such signature". — Sheen
– Sheen, Commented Jan 15, 2014 at 14:14
It's absolutely fine in the vast majority of cases where I've wanted to use generics, because in that vast majority of cases I haven't wanted to call operator+ - it's as simple as that. You say that C# "needs" some contract specification - you might like it (and so might I) but that's not the same as "needing" it. — Jon Skeet
– Jon Skeet, Commented Jan 15, 2014 at 14:21

Jon Skeet · Accepted Answer · 2014-01-15 13:58:50Z

14

Is this possible? If not, it means C# generics is very limited in use I think.

Even if it weren't possible, generics can be used in many many situations.

However, it's really simple given that all the primitive types implement IComparable<T>;

void Foo<T>(IList<T> list) where T : IComparable<T>
{
    T x = ...;
    T y = ...;
    if (x.CompareTo(y) < 0)
    {
        ...
    }
}

Note that this will not introduce boxing operations. I'd expect the JITted code for primitive types to end up having very similar performance to hardcoding it to use < or >.

Now this is a slightly special case, because there's an interface which is roughly equivalent to those operators. What isn't as easy is using arithmetic operators (+, - etc). See the article on my web site for a few approaches to this.

answered Jan 15, 2014 at 13:58

Jon Skeet

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2 Comments

Sheen Over a year ago

Thanks, @Jon Skeet You answered to my question and also confirmed that C# generics is not as flexible and powerful as C++ template.

Jon Skeet Over a year ago

@Sheen: They're not the same, and weren't meant to be the same. There are benefits of each approach, and each is suitable for the design of the language it's part of.

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