Sort a list of interface objects

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>?

Kickshaw

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

Dour

This works well, since sorting by an arbitrary column is a requirement. It also encapsulates the sort quite well, which makes for more readable code. Having a generic property comparer is quite useful for other purposes as well. – Kickshaw

Answer 2

I am not sure what problem you are running into because I just ran a quick test and could sort a list of IFoo. See below for how I did this. If this does not fit your need, could you maybe give more detail?

var fooList = new List<IFoo>{new testFoo{key=3}, new testFoo{key=1}};
fooList.Sort(
    delegate(IFoo obj1, IFoo obj2){return obj1.key.CompareTo(obj2.key);});

Interface and Concrete

public interface IFoo
{
     int key{get;set;}
}

public class testFoo:IFoo
{
    public int key {get;set;}
}

Coenzyme

Answer 3

If you are using C# 3/4, you can use a lambda expression..

This example shows how you can sort by different properties of the IFoo interface:

void Main()
{
    List<IFoo> foos = new List<IFoo>
    {
        new Bar2{ Name = "Pqr", Price = 789.15m, SomeNumber = 3 },
        new Bar2{ Name = "Jkl", Price = 444.25m, SomeNumber = 1 },
        new Bar1{ Name = "Def", Price = 222.5m, SomeDate = DateTime.Now },
        new Bar1{ Name = "Ghi", Price = 111.1m, SomeDate = DateTime.Now },
        new Bar1{ Name = "Abc", Price = 123.45m, SomeDate = DateTime.Now },
        new Bar2{ Name = "Mno", Price = 564.33m, SomeNumber = 2 }

    };

    foos.Sort((x,y) => x.Name.CompareTo(y.Name));
    foreach(IFoo foo in foos)
    {
        Console.WriteLine(foo.Name);
    }

    foos.Sort((x,y) => x.Price.CompareTo(y.Price));
    foreach(IFoo foo in foos)
    {
        Console.WriteLine(foo.Price);
    }
}

interface IFoo
{
    string Name { get; set; }
    decimal Price { get; set; }
}

class Bar1 : IFoo
{
    public string Name { get; set; }
    public decimal Price { get; set; }
    public DateTime SomeDate { get; set; }
}

class Bar2 : IFoo
{
    public string Name { get; set; }
    public decimal Price { get; set; }
    public int SomeNumber { get; set; }
}

Output:

Abc
Def
Ghi
Jkl
Mno
Pqr
111.1
222.5
333.33
444.25
555.45
666.15

Matchwood