Enumerable IsOrderedSubsetOf

EnumerableExtensions.cs

public static class EnumerableExtensions
{
    public static bool IsOrderedSubsetOf<T>(
        this IEnumerable<T> source,
        IEnumerable<T> target,
        out T missing,
        IEqualityComparer<T> equalityComparer = null)
    {
        if (null == equalityComparer)
            equalityComparer = EqualityComparer<T>.Default;

        using (var primary = source.GetEnumerator())
        using (var secondary = target.GetEnumerator())
        {
            while (true)
            {
                var moveNext = primary.MoveNext();
                if (!moveNext)
                {
                    // empty set is always a subset
                    missing = default;
                    return true;
                }

                if (!secondary.MoveNext())
                {
                    // we have a value in a, and didn't see it in b
                    missing = primary.Current;
                    return false;
                }

                while (!equalityComparer.Equals(primary.Current, secondary.Current))
                {
                    if (secondary.MoveNext())
                        continue;

                    // we have a value in a, and didn't see it in b
                    missing = primary.Current;
                    return false;
                }
            }
        }
    }
}