Generic IEqualityComparer and GetHashCode

Question

Being somewhat lazy about implementing lots of IEqualityComparers, and given that I couldn\'t easily edit class implementations of object being compared, I went with the fol

Answer 1

If a common use-case is comparing objects according to one of their properties, you could add an additional constructor and implement and call it like this:

public GenericEqualityComparer(Func projection)
{
    compareFunction = (t1, t2) => projection(t1).Equals(projection(t2));
    hashFunction = t => projection(t).GetHashCode();
}

var comaparer = new GenericEqualityComparer( o => o.PropertyToCompare);

This will automatically use the hash as implemented by the property.

EDIT: a more efficient and robust implementation inspired my Marc's comments:

public static GenericEqualityComparer Create(Func projection)
{
    return new GenericEqualityComparer(
        (t1, t2) => EqualityComparer.Default.Equals( projection(t1), projection(t2)),
        t => EqualityComparer.Default.GetHashCode(projection(t)));
}

var comparer = GenericEqualityComparer.Create( o => o.PropertyToCompare);