Three years ago I blogged about the “iPhone bringing the Internet to your pocket”.  And now we have the iPad, the target device for most innovation in tablet apps.

If, like me, you’re keen to build iOS apps but are primarily a Microsoft developer and are daunted by the learning curve of a whole new platform, then MonoTouch might be for you: http://monotouch.net/

MonoTouch is a bridge between .NET and the native iOS API called CocoaTouch.  It exposes the CocoaTouch as a .NET library so you can write code in C# instead of Objective-C. This gives you access to all the native CocoaTouch GUI widgets and a whole bunch of regular .NET libraries.

This is really neat!  Have a look at the Hello World tutorial to get a better feel of MonoTouch: http://monotouch.net/Tutorials/MonoDevelop_HelloWorld

I was recently doing some work where I had to use Except and Insersect, but with some custom comparison logic.  Let me illustrate by way of an example.  If I had the following class

public class Widget
{
    public int ID { get; set; }
    public string Name { get; set; }
}

and created two lists like this

List<Widget> first = new List<Widget>()
{
    new Widget() { ID = 1, Name = "Alpha"},
    new Widget() { ID = 2, Name = "Beta"},
    new Widget() { ID = 3, Name = "Gamma"},
}; 

List<Widget> second = new List<Widget>()
{
    new Widget() { ID = 2, Name = "Foo"},
    new Widget() { ID = 3, Name = "Bar"},
    new Widget() { ID = 4, Name = "Baz"},
};

and tried to use Except like this

first.Except(second).ToList().ForEach(x => Console.WriteLine("ID=" + x.ID + ", " + x.Name));

I would get:

ID=1, Alpha
ID=2, Beta
ID=3, Gamma

Since I wanted to do a comparison on the ID property, what I really want is just the first item the list i.e. [ID=1, Alpha].  One way of achieving this to override Equals() on the Widget class, but this may not always be practical or even possible.  Another option is to supply a custom IEqualityComparer<T>.  To do this I would need to a define a class that implements Equals() and GetHashCode(), the two methods on IEqualityComparer<T> and supply an instance of this class as an additional parameter to Equals() or Intersect().  Unfortunately, I found neither of these practical since I had a number of classes that that I wanted to use with Except() or Intersect() and wanted a more elegant solution.

“Why not just use a lambda expression to express the comparison?”, I thought and came up with the following two extensions:

public static class LinqExtensions
{
    public static IEnumerable<TSource> Except<TSource>(this IEnumerable<TSource> first, IEnumerable<TSource> second, Func<TSource, TSource, bool> comparer)
    {
        return first.Where(x => second.Count(y => comparer(x, y)) == 0);
    } 

    public static IEnumerable<TSource> Intersect<TSource>(this IEnumerable<TSource> first, IEnumerable<TSource> second, Func<TSource, TSource, bool> comparer)
    {
        return first.Where(x => second.Count(y => comparer(x, y)) == 1);
    }
}

Now I can compare on the ID by supplying the comparison logic as a lambda expression:

Console.WriteLine("Except");
first.Except(second, (x, y) => x.ID == y.ID).ToList().ForEach(x => Console.WriteLine("ID=" + x.ID + ", " + x.Name));

Console.WriteLine("Intersect");
first.Intersect(second, (x, y) => x.ID == y.ID).ToList().ForEach(x => Console.WriteLine("ID=" + x.ID + ", " + x.Name));

This gives me the results I want:

Except
ID=1, Alpha
Intersect
ID=2, Beta
ID=3, Gamma

Hope you find them useful!