Using LINQ to objects Intersect and Except on a specific property

Question

When I have 2 List<string> objects, then I can use Intersect and Except on them directly to get an output IEnumerable<string>. That's simple enough, but what if I want the intersection/disjuction on something more complex?

Example, trying to get a collection of ClassA objects which is the result of the intersect on ClassA object's AStr1 and ClassB object's BStr; :

public class ClassA {
    public string AStr1 { get; set; }
    public string AStr2 { get; set; }
    public int AInt { get; set; }
}
public class ClassB {
    public string BStr { get; set; }
    public int BInt { get; set; }
}
public class Whatever {
    public void xyz(List<ClassA> aObj, List<ClassB> bObj) {
        // *** this line is horribly incorrect ***
        IEnumberable<ClassA> result =
            aObj.Intersect(bObj).Where(a, b => a.AStr1 == b.BStr);
    }
}

How can I fix the noted line to achieve this intersection.

Answer 1

MoreLINQ has ExceptBy. It doesn't have IntersectBy yet, but you could easily write your own implementation, and possibly even contribute it to MoreLINQ afterwards :)

It would probably look something like this (omitting error checking):

    public static IEnumerable<TSource> IntersectBy<TSource, TKey>(
        this IEnumerable<TSource> first,
        IEnumerable<TSource> second,
        Func<TSource, TKey> keySelector,
        IEqualityComparer<TKey> keyComparer)
    {
        HashSet<TKey> keys = new HashSet<TKey>(first.Select(keySelector),
                                               keyComparer);
        foreach (var element in second)
        {
            TKey key = keySelector(element);
            // Remove the key so we only yield once
            if (keys.Remove(key))
            {
                yield return element;
            }
        }
    }

If you wanted to perform an intersection on two completely different types which happened to have a common property type, you could make a more general method with three type parameters (one for first, one for second, and one for the common key type).

Answer 2

x ∈ A ∩ B if and only if x ∈ A and x ∈ B.

So, for each a in aObj, you can check if a.AStr1 is in the set of BStr values.

public void xyz(List<ClassA> aObj, List<ClassB> bObj)
{
    HashSet<string> bstr = new HashSet<string>(bObj.Select(b => b.BStr));
    IEnumerable<ClassA> result = aObj.Where(a => bstr.Contains(a.AStr1));
}

Answer 3

this code:

    public IEnumerable<ClassA> xyz(List<ClassA> aObj, List<ClassB> bObj)
    {
        IEnumerable<string> bStrs = bObj.Select(b => b.BStr).Distinct();
        return aObj.Join(bStrs, a => a.AStr1, b => b, (a, b) => a);
    }

has passed the following test:

    [TestMethod]
    public void PropertyIntersectionBasedJoin()
    {
        List<ClassA> aObj = new List<ClassA>()
                                {
                                    new ClassA() { AStr1 = "a" }, 
                                    new ClassA() { AStr1 = "b" }, 
                                    new ClassA() { AStr1 = "c" }
                                };
        List<ClassB> bObj = new List<ClassB>()
                                {
                                    new ClassB() { BStr = "b" }, 
                                    new ClassB() { BStr = "b" }, 
                                    new ClassB() { BStr = "c" }, 
                                    new ClassB() { BStr = "d" }
                                };

        var result = xyz(aObj, bObj);

        Assert.AreEqual(2, result.Count());
        Assert.IsFalse(result.Any(a => a.AStr1 == "a"));
        Assert.IsTrue(result.Any(a => a.AStr1 == "b"));
        Assert.IsTrue(result.Any(a => a.AStr1 == "c"));
    }