Combining 2 lists and and remove duplicates .Output in a third list .My attempts do not work

Question

I always seem to have a problem when I need to compare 2 list and produce a 3rd list which include all unique items.I need to perform this quite often.

Attempt to re

Answer 1

I had a similar problem where I had two very large lists with random strings.

I made a recursive function which returns a new list with unique strings. I compared two lists with 100k random strings(it may or may not exist duplicates) each with 6 characters of abcdefghijklmnopqrstuvwxyz1234567890 and it was done in about 230 ms. I only measured the given function.

I hope this will give value to someone.

Image of test run

makeCodesUnique(List existing, List newL)
{
    // Get all duplicate between two lists
    List duplicatesBetween = newL.Intersect(existing).ToList();

    // Get all duplicates within list
    List duplicatesWithin = newL.GroupBy(x => x)
    .Where(group => group.Count() > 1)
    .Select(group => group.Key).ToList();

    if (duplicatesBetween.Count == 0 && duplicatesWithin.Count == 0)
    {
        // Return list if there are no duplicates
        return newL; 
    }
    else
    {
        if (duplicatesBetween.Count != 0)
        {
            foreach (string duplicateCode in duplicatesBetween)
            {
                newL.Remove(duplicateCode);
            }

            // Generate new codes to substitute the removed ones
            List newCodes = generateSomeMore(duplicatesBetween.Count);
            newL.AddRange(newCodes);
            makeCodesUnique(existing, newL);
        }
        else if (duplicatesWithin.Count != 0)
        {
            foreach (string duplicateCode in duplicatesWithin)
            {
                newL.Remove(duplicateCode);
            }
            List newCodes = generateSomeMore(duplicatesWithin.Count);
            new.AddRange(newCodes);
            makeCodesUnique(existing, newL);
        }
    }
    return newL;
}