Question

Can I write these functions more efficient?

public static void AddDistinct<T>(this ICollection<T> source, params ICollection<T>[] collections)
{
    (from collection in collections from item in collection where !source.Contains(item) select item).ForEach(source.Add);
}

public static void AddDistinct<T>(this ICollection<T> source, Func<T, bool> predicate, params ICollection<T>[] collections)
{
    collections.ForEach(collection => collection.Where(predicate).Where(item => !source.Contains(item)).ForEach(source.Add));
}

public static void AddDistinct<T>(this ICollection<T> source, params T[] items)
{
    items.Where(item => !source.Contains(item)).ForEach(source.Add);
}

public static void AddDistinct<T>(this ICollection<T> source, Func<T, bool> predicate, params T[] items)
{
    items.Where(predicate).Where(x => !source.Contains(x)).ForEach(source.Add);
}

Answer 1

You can use the Except method:

public static void AddDistinct<T>(this ICollection<T> source, params ICollection<T>[] collections)
{
    var itemsToAdd = collections.SelectMany(x => x).Where(predicate).Except(source).ToArray();
    itemsToAdd.ForEach(source.Add));
}

public static void AddDistinct<T>(this ICollection<T> source, Func<T, bool> predicate, params ICollection<T>[] collections)
{
    var itemsToAdd = collections.SelectMany(x => x).Where(predicate).Except(source).ToArray();
    itemsToAdd.ForEach(source.Add));
}

public static void AddDistinct<T>(this ICollection<T> source, params T[] items)
{
    var itemsToAdd = items.Except(source).ToArray();
    itemsToAdd.ForEach(source.Add));
}

public static void AddDistinct<T>(this ICollection<T> source, Func<T, bool> predicate, params T[] items)
{
    var itemsToAdd = items.Where(predicate).Except(source).ToArray();
    itemsToAdd.ForEach(source.Add));
}

Note that because Where and Except have deferred and streamed execution, you need the ToArray() call to ensure the enumeration of source is complete before you add anything to it (since a collection can’t be modified while it’s being enumerated).