Create a Dictionary from another Dictionary and a List<T> on a matching values

Question

I have the following function, which takes a Dictionary and a List, matches rows in each of those and returns another Dictionary based on matcing items.

Is there a better way (code and performance -wise) to achieve the same result?

    public sealed class ProdIds 
    {
             public List<ProdID> Items { get; set; }
    }
    
    public sealed class ProdID
    {
            public string SpecialId { get; set; }
            public int ItemId { get; set; }
    }
    

Simplified view of the entries:

names: {100, "Name1"}, {333, "Name3"}, {212, "Name55"}, {99, "NameABC"}, ...
ids:  {"SP44", 212}, {"SP33", 333}, {"SP11", 100}, {"SP9", 99}, ...
    
    
private static Dictionary<string, string> CreateMatchedDictionary (IReadOnlyDictionary<int, string> names, List<ProdIds> ids)
{
    var dic = new Dictionary<string, string>();
    foreach (var name in names)
    {
        foreach (var id in ids)
        {
            if (name.Key == id.Items[0].ItemId)
            {
                dic.Add(id.Items[0].SpecialId, name.Value);
            }
        }
    }
    return dic;
}

What I want to be returned here, is a new Dictionary which would look similar to this:

dic: {"SP44", "Name55"}, {"SP33", "Name3"}, {"SP11", "Name1"}, {"SP9", "NameABC"}, ...

Answer 1

The main performance problem is that you're looping through the names dictionary instead of taking advantage of the built-in O(1) lookup:

private static Dictionary<string, string> CreateMatchedDictionary (IReadOnlyDictionary<int, string> names, List<ProdIds> ids)
{
    var dic = new Dictionary<string, string>();
    foreach (var id in ids)
    {
        string name = null;
        if (names.TryGetValue(id.Items[0].ItemId, out name)
        {
            dic.Add(id.Items[0].SpecialId, name);
        }
    }
    return dic;
}

You could use Linq to make the code more concise, but it's not going to improve performance and might make debugging harder.