What's the recommended best practice for using IEqualityComparer?

Question

I\'m looking for real world best practices, how other people might have implemented solutions with complex domains.

Answer 1

Any time you consider using an IEqualityComparer, pause to think if the class could be made to implement IEquatable instead. If a Product should always be compared by ID, just define it to be equated as such so you can use the default comparer.

That said, there are still a few of reasons you might want a custom comparer:

1. If there are multiple ways instances of a class could be considered equal. The best example of this is a string, for which the framework provides six different comparers in StringComparer.
2. If the class is defined in such a way that you can't define it as IEquatable. This would include classes defined by others and classes generated by the compiler (specifically anonymous types, which use a property-wise comparison by default).

If you do decide you need a comparer, you can certainly use a generalized comparer (see DMenT's answer), but if you need to reuse that logic you should encapsulate it in a dedicated class. You could even declare it by inheriting from the generic base:

class ProductByIdComparer : GenericEqualityComparer
{
    public ProductByIdComparer()
        : base((x, y) => x.ProductId == y.ProductId, z => z.ProductId)
    { }
}

As far as use, you should take advantage of comparers when possible. For example, rather than calling ToLower() on every string used as a dictionary key (logic for which will be strewn across your app), you should declare the dictionary to use a case-insensitive StringComparer. The same goes for the LINQ operators that accept a comparer. But again, always consider if the equatable behavior that should be intrinsic to the class rather than defined externally.