Question

I have a Metamodel that’s built like this:

class ModelElement
{
    string id;
}

class Package : ModelElement
{
     List<Package> nestedPackages;
     List<Class> ownedClasses;
}

class Class : ModelElement
{
}

Now I’ve built two Models and I want to check if they’re identical. I’d like to compare the ID’s of the Elements and I don’t want to write a method for any type of Element.

Package a; //pretend both have classes
Package b; //and nested packages
compare(a.nestedPackages, b.nestedPackages);
compare(a.ownedClasses; b.OwnedClasses);

Since Class and Package both inherit from ModelElement, both have IDs. So I want to write a Function “compare” which compares the IDs. I thought of using Generics but the generic datatype doesn’t have the attribute “id”. Any ideas?

Answer 1

You could look at Enumerable.SequenceEqual, combined with a custom comparer.

bool equal = Enumerable.SequenceEqual(a.nestedPackages, b.nestedPackages,
    new ModelElementComparer());

public class ModelElementComparer : IEqualityComparer<ModelElement>
{
    public bool Equals(ModelElement x, ModelElement y)
    {
        return x.id == y.id;
    }

    public int GetHashCode(ModelElement obj)
    {
        return x.id;
    }
}

Here are MSDN links to this particular SequenceEqual override and the IEqualityComparer interface.