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.

Answer 1

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 .

Answer 2

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.

Answer 3

你必须使用IComparable<T>的CompareTo方法。

Answer 4

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.

Answer 5

It is not directly possible, but there are workarounds.

I posted mine in a different question here.