Getting matching items in lists based on nullable properties

I'm iterating through two lists to find matching objects. The objects have two string properties, Id1 and Id2 below. If Id1 is not null, empty or whitespace, I want to use it to find a matching object in the other list. Otherwise I want to make use of Id2.

If both Id1 and Id2 are set to null by the extension method on either p or p2, I want to skip the iteration, since I know that I won't find a match.

public static string NullIfEmptyOrWhiteSpace(this string value)
{
    return string.IsNullOrEmpty(value) ? null : value;
}

foreach (Product p2 in productList1)
{ 
    Product match = productList2.FirstOrDefault(p =>
    {
        string pId = p.Id1.NullIfEmptyOrWhiteSpace() ??
                     p.Id2.NullIfEmptyOrWhiteSpace();

        string p2Id = p2.Id1.NullIfEmptyOrWhiteSpace() ??
                      p2.Id2.NullIfEmptyOrWhiteSpace();

        if (pId == null || p2Id == null)
            return false;

        return pId.Equals(p2Id);
    });

    // do stuff with match
}

This seems to work, but it feels like I'm re-inventing the wheel here. Are there any built in methods that I can make use of to simplify the code? Perhaps a class that implements IEqualityComparer?

Example use case:

List list1 = new List()
{  
    new Product() { Id1 = "123", Id2 = null },
    new Product() { Id1 = "435", Id2 = null },
    new Product() { Id1 = null, Id2 = "123" }
}

List list12 = new List()
{  
    new Product() { Id1 = "123", Id2 = null },
    new Product() { Id1 = "456", Id2 = null },
    new Product() { Id1 = null, Id2 = "123" }
}

// do something with each matching item in the first list
// which would be item #1 and item #3 above

Let's start refactoring!

foreach (Product p2 in productList1)
{ 
    Product match = productList2.FirstOrDefault(p =>
    {
        string pId = p.Id1.NullIfEmptyOrWhiteSpace() ??
                     p.Id2.NullIfEmptyOrWhiteSpace();

        string p2Id = p2.Id1.NullIfEmptyOrWhiteSpace() ??
                      p2.Id2.NullIfEmptyOrWhiteSpace();

        if (pId == null || p2Id == null)
            return false;

        return pId.Equals(p2Id);
    });

    // do stuff with match
}

p2Id does not depend on p, so let's move it out of the lambda.

foreach (Product p2 in productList1)
{ 
    string p2Id = p2.Id1.NullIfEmptyOrWhiteSpace() ??
                  p2.Id2.NullIfEmptyOrWhiteSpace();

    Product match = productList2.FirstOrDefault(p =>
    {
        string pId = p.Id1.NullIfEmptyOrWhiteSpace() ??
                     p.Id2.NullIfEmptyOrWhiteSpace();

        if (pId == null || p2Id == null)
            return false;

        return pId.Equals(p2Id);
    });

    // do stuff with match
}

Now let's define a method on Product to remove the repetition. I called it GetKey, but there's probably a better name.

public string GetKey()
{
    return this.Id1.NullIfEmptyOrWhiteSpace() ?? this.Id2.NullIfEmptyOrWhiteSpace();
}

foreach (Product p2 in productList1)
{ 
    string p2Id = p2.GetKey();

    Product match = productList2.FirstOrDefault(p =>
    {
        string pId = p.GetKey();

        if (pId == null || p2Id == null)
            return false;

        return pId.Equals(p2Id);
    });

    // do stuff with match
}

It looks like you actually want to call IsNullOrWhiteSpace in NullIfEmptyOrWhiteSpace, so let's fix that.

public static string NullIfEmptyOrWhiteSpace(this string value)
{
    return string.IsNullOrWhiteSpace(value) ? null : value;
}

Actually, we don't need to go through productList2 if p2Id is null.

foreach (Product p2 in productList1)
{ 
    string p2Id = p2.GetKey();
    if (p2Id == null)
    {
        continue;
    }

    Product match = productList2.FirstOrDefault(p =>
    {
        string pId = p.GetKey();

        if (pId == null)
            return false;

        return pId.Equals(p2Id);
    });

    // do stuff with match
}

Given that we only use p2 for its key, we can rewrite this as

foreach (var key in productList1.Select(product => product.GetKey()).Where(key => key != null))
{
    Product match = productList2.FirstOrDefault(p =>
    {
        string pId = p.GetKey();

        if (pId == null)
            return false;

        return pId.Equals(key);
    });

    // do stuff with match
}

Now let's use a dictionary to replace the call to productList2.FirstOrDefault.

var firstProductWithKey = new Dictionary();
foreach (var product in productList2)
{
    var key = product.GetKey();
    if (key != null && !firstProductWithKey.ContainsKey(key))
    {
        firstProductWithKey.Add(key, product);
    }
}

if (!firstProductWithKey.Any())
{
    return;
}

foreach (var key in productList1.Select(product => product.GetKey()).Where(key => key != null))
{
    Product match;
    if (firstProductWithKey.TryGetValue(key, out match))
    {
        // do stuff with match
    }
}

So what have we gained? Hopefully the intention of the code is a bit clearer, but there's also a performance benefit.

Let's say that the lists have \$n\$ and \$m\$ elements respectively, and that there are no keys common to the two lists.

In the original code, we're evaluating each key in the first list \$m\$ times, and each key in the second list \$n\$ times. So \$2nm\$ keys in total.

In the refactored version, we evaluate each key in the first list once, and each key in the second list once, for \$n + m\$ keys. We're performing much less work, and generating much less garbage for the GC.

StackExchange Code Review Q#55778, answer score: 6