C# Generic List Union Question

Question

I'm trying to merge 2 lists using "Union" so I get rid of duplicates. Following is the sample code:

public class SomeDetail
{
    public string SomeValue1 { get; set; }
    public string SomeValue2  { get; set; }
    public string SomeDate { get; set; }
}

public class SomeDetailComparer : IEqualityComparer<SomeDetail>
{
    bool IEqualityComparer<SomeDetail>.Equals(SomeDetail x, SomeDetail y)
    {
        // Check whether the compared objects reference the same data.        
        if (Object.ReferenceEquals(x, y))
            return true;
        // Check whether any of the compared objects is null.        
        if (Object.ReferenceEquals(x, null) || Object.ReferenceEquals(y, null))
            return false;
        return x.SomeValue1 == y.SomeValue1 && x.SomeValue2 == y.SomeValue2;
    }
    int IEqualityComparer<SomeDetail>.GetHashCode(SomeDetail obj)
    {
        return obj.SomeValue1.GetHashCode();
    }
}

List<SomeDetail> tempList1 = new List<SomeDetail>();
List<SomeDetail> tempList2 = new List<SomeDetail>();

List<SomeDetail> detailList = tempList1.Union(tempList2, SomeDetailComparer).ToList();

Now the question is can I use Union and still get the record which has the latest date (using SomeDate property). The record itself can either be in tempList1 or tempList2.

Thanks in advance

Answer 1

You'd have to be able to tell Union how to pick which one of the duplicates to use. I don't know of a way to do that other than writing your own Union.

Answer 2

The operation that is really suited to this purpose is an full outer join. The Enumerable class has an implementation of inner join, which you can use to find the duplicates and select whichever you prefer.

var duplicates = Enumerable.Join(tempList1, tempList2,  keySelector, keySelector, 
    (item1, item2) => (item1.SomeDate > item2.SomeDate) ? item1 : item2)
    .ToList();

keySelector is simply a function (could be a lambda expression) that extracts a key from an object of type SomeDetail. Now, to implement the full outer join, try something like this:

var keyComparer = (SomeDetail item) => new { Value1 = item.SomeValue1,
    Value2 = item.SomeDetail2 };
var detailList = Enumerable.Union(tempList1.Except(tempList2, equalityComparer), 
    tempList2.Except(tempList1, equalityComparer)).Union(
    Enumerable.Join(tempList1, tempList2, keyComparer, keyComparer
    (item1, item2) => (item1.SomeDate > item2.SomeDate) ? item1 : item2))
    .ToList();

equalityComparer should be an object that implements IEqualityComparer<SomeDetail> and effectively uses the keyComparer function for testing equality.

Let me know if that does the job for you.

Answer 3

You cannot with the standard Union method, but you can create an extension method Union for List<SomeDetail> with this special handling and this method will be used because the signature fits better.

Answer 4

Why not just use HashSet<T>?

List<SomeDetail> tempList1 = new List<SomeDetail>();
List<SomeDetail> tempList2 = new List<SomeDetail>();

HashSet<SomeDetail> hs = new HashSet<SomeDetail>(new SomeDetailComparer());

hs.UnionWith(tempList1);
hs.UnionWith(tempList2);

List<SomeDetail> detailList = hs.ToList();

Answer 5

Merge generic lists

    public static List<T> MergeListCollections<T>(List<T> firstList, List<T> secondList)
    {
        List<T> merged = new List<T>(firstList);
        merged.AddRange(secondList);
        return merged;
    }