Question

I don’t understand what is going on here…

I’ve got the following error:
The type 'TestApp.TestVal' cannot be used as type parameter 'T' in the generic type or method 'TestApp.SomeClass<T>'. There is no boxing conversion from 'TestApp.TestVal' to 'System.IComparable<TestApp.TestVal>'.

This error happens for the following code:

public enum TestVal
{
    First,
    Second,
    Third
}

public class SomeClass<T>
    where T : IComparable<T>
{
    public T Stored
    {
        get
        {
            return storedval;
        }
        set
        {
            storedval = value;
        }
    }
    private T storedval;
}

class Program
{
    static void Main(string[] args)
    {
        //Error is on the next line
        SomeClass<TestVal> t = new SomeClass<TestVal>(); 
    }
}

Since the enum is an int by default and int’s implement the IComparable<int> interface it seems like there shouldn’t be an error….

Answer 1

Firstly, I’m not sure whether it is sensible to use IComparable<T> with an enum… IEquatable<T>, sure – but comparison?

As a safer alternative; rather than mandate the IComparable<T> with the generic constraint, perhaps use Comparer<T>.Default inside the class. This has the advantage of supporting IComparable<T> and IComparable – and it means you have less constraints to propagate.

For example:

public class SomeClass<T> { // note no constraint
    public int ExampleCompareTo(T other) {
        return Comparer<T>.Default.Compare(Stored, other);
    }
    ... [snip]
}

This works fine with the enum:

SomeClass<TestVal> t = new SomeClass<TestVal>();
t.Stored = TestVal.First;
int i = t.ExampleCompareTo(TestVal.Second); // -1