Question

i have what seems like a common problem / pattern. two collections of the same object. The object has a number of properties and some nested objects within it. Car has a property called id which is the unique identifier.

I want to find the LINQ way to do a diff, which includes:

1. Items in one collection and not the other (visa versa)
2. For the items that match, are there any changes (changes would be a comparison of all properties? (i only care about settable properties, would i use reflection for this ?? )

Answer 1

You can use the Enumerable.Except() method. This uses a comparer (either a default or one you supply) to evaluate which objects are in both sequences or just one:

var sequenceA = new[] { "a", "e", "i", "o", "u" };
var sequenceB = new[] { "a", "b", "c" };

var sequenceDiff = sequenceA.Except( sequenceB );

If you want to perform a complete disjunction of both sequences (A-B) union (B-A), you would have to use:

var sequenceDiff = 
         sequenceA.Except( sequenceB ).Union( sequenceB.Except( sequenceA ) );

If you have a complex type, you can write an IComparer<T> for your type T and use the overload that accepts the comparer.

For the second part of your question, you would need to roll your own implementation to report which properties of a type are different .. there’s nothing built into the .NET BCL directly. You have to decide what form this reporting would take? How would you identify and express differences in a complex type? You could certainly use reflection for this … but if you’re only dealing with a single type I would avoid that, and write a specialized differencing utility just for it. If yo’re going to support a borad range of types, then reflection may make more sense.