Binary comparison operators on generic types

Question

I have a generic class that takes a type T. Within this class I have a method were I need to compare a type T to another type T such as:

public class MyClass<T>
{
    public T MaxValue
    {
        // Implimentation for MaxValue
    }

    public T MyMethod(T argument)
    {
        if(argument > this.MaxValue)
        {
             // Then do something
        }
    }
}

The comparison operation inside of MyMethod fails with Compiler Error CS0019. Is it possible to add a constraint to T to make this work? I tried adding a where T: IComparable<T> to the class definition to no avail.

It's Impossible =( . However there is an open source math library that tries to deal with it, have to recall the name... — vittore, Mar 31 '10 at 19:16

score 9 · Accepted Answer · edited May 23 '17 at 11:47

Adding constraint to make sure that the type implements IComparable<T> is a way to go. However, you cannot use the < operator - the interface provides a method CompareTo to do the same thing:

public class MyClass<T> where T : IComparable<T> { 
    public T MaxValue  { 
        // Implimentation for MaxValue 
    } 

    public T MyMethod(T argument) { 
        if(argument.CompareTo(this.MaxValue) > 0){ 
             // Then do something 
        } 
    } 
}

If you needed other numeric operators than just comparison, the situation is more difficult, because you cannot add constraint to support for example + operator and there is no corresponding interface. Some ideas about this can be found here.

Good call Tomas. I found a blog post at http://blogs.telerik.com/hristoborisov/posts/09-01-10/comparing_generic_types.aspx regarding this exact issue shortly after I asked the question that confirms what you are saying. — Brian Triplett, Mar 31 '10 at 19:18

score 3 · Answer 2 · answered Mar 31 '10 at 19:16

3

No, it is not possible, operator constraints are unfortunately not allowed in c#. where T: IComparable<T> should work just fine, but you have to use other semantics: CompareTo method instead of >,< and == operators.

answered Mar 31 '10 at 19:16

n535

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Yes and I'm not sure the OP is married to the use of operators for this. – Mar 31 '10 at 19:17
Hmm..I don't really understood your comment. – n535 Mar 31 '10 at 19:20
he meant that the OP is not obliged to use those operators. He can use any other way, als long is it works. – Pim Jager Mar 31 '10 at 19:23

score 2 · Answer 3 · answered Mar 31 '10 at 19:16

2

You have to use IComparable<T>'s CompareTo method.

answered Mar 31 '10 at 19:16

score 1 · Answer 4 · answered Mar 31 '10 at 21:08

Actually, you don't need anything special here. Comparer<T>.Default offers a .Compare method that will do everything you need without requiring any additional generic constraints (which tend to propagate horribly). It works for both IComparable<T> and IComparable, and supports classes, structs and Nullable<T>. Job done.

For info, this is how the LINQ Max / Min implementations work, and likewise List<T>.Sort. So a common and well supported part of the BCL.

score 0 · Answer 5 · edited May 23 '17 at 10:29

0

It is not directly possible, but there are workarounds.

I posted mine in a different question here.

edited May 23 '17 at 10:29

Community

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answered Mar 31 '10 at 19:21

Lucero

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Binary comparison operators on generic types

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