Merging two IENumerable<T>s

Question

I have two IEnumerable's. One gets filled with the fallback ellements. This one will always contain the most elements. The other one will get filled depending on some parameters and will possibly contain less elements. If an element doesn't exist in the second one, I need to fill it with the equivalent one of the first one.

This code does the job, but feels inefficient to me and requires me to cast the IEnumerables to ILists or to use a temporary list Person implements IEquatable

IEnumerable<Person> fallBack = Repository.GetPersons();
IList<Person> translated = Repository.GetPersons(language).ToList();

foreach(Person person in fallBack){
    if(!translated.Any(p=>p.equals(person)))
        translated.add(person);  
}

Any suggestions?

Answer 1

translated.Union(fallback)

or (if Person doesn't implement IEquatable<Person> by ID)

translated.Union(fallback, PersonComparer.Instance)

where PersonComparer is:

public class PersonComparer : IEqualityComparer<Person>
{
    public static readonly Instance = new PersonComparer();

    // We don't need any more instances
    private PersonComparer() {}

    public int GetHashCode(Person p)
    {
        return p.id;
    }

    public bool Equals(Person p1, Person p2)
    {
        if (Object.ReferenceEquals(p1, p2))
        {
            return true;
        }
        if (Object.ReferenceEquals(p1, null) ||
            Object.ReferenceEquals(p2, null))
        {
            return false;
        }
        return p1.id == p2.id;
    }
}

Answer 2

Try this.

public static IEnumerable<Person> SmartCombine(IEnumerable<Person> fallback, IEnumerable<Person> translated) {
  return translated.Concat(fallback.Where(p => !translated.Any(x => x.id.equals(p.id)));
}