Question

I have a couple of classes that implement an interface, IFoo. I need to display a list of objects of these classes in a table, which I’d like to be able to sort by any arbitrary column in the table. So, the data source for the table is a List<IFoo>.

The problem I’ve run into is that the standard way of implementing IComparable or IComparer for objects that are used in a list requires a static method, but static methods aren’t allowed in interfaces. So, the question boils down to this: how does one sort a List<IFoo>?

Answer 1

IComparable

I don’t know what gives you the idea that you need to be using a static method, but it’s not correct.

You can force all your IFoo implementers to implement IComparable<IFoo> by adding it to IFoo:

interface IFoo : IComparable<IFoo> 
{ 
    int Value { get; set; } // for example's sake
}

class SomeFoo : IFoo
{
    public int Value { get; set; }

    public int CompareTo(IFoo other)
    {
        // implement your custom comparison here...

        return Value.CompareTo(other.Value); // e.g.
    }
}

Then simply sort your List<IFoo> like so:

list.Sort();

Sorting by an arbitrary column

You initially stated you want to sort by an arbitrary column in a table of IFoo objects. This is more complex; you need to be able to sort a list of objects by any one of their public properties, so the basic IComparable<IFoo> implementation above isn’t going to cut it.

The solution is to create a PropertyComparer<T> class which implements IComparer<T>, and will sort by any property of T. You could write it specifically for IFoo, but at some point every developer tends to come up against this problem and ends up writing a generic property compararer that will try to sort any type. As a result you can google “c# property comparer” and you’re sure to get several hits. Here’s a simple one:

http://www.codeproject.com/Articles/16200/Simple-PropertyComparer