C# LINQ find duplicates in List

Question

Using LINQ, from a List, how can I retrieve a list that contains entries repeated more than once and their values?

Answer 1

You can do this:

var list = new[] {1,2,3,1,4,2};
var duplicateItems = list.Duplicates();

With these extension methods:

public static class Extensions
{
    public static IEnumerable Duplicates(this IEnumerable source, Func selector)
    {
        var grouped = source.GroupBy(selector);
        var moreThan1 = grouped.Where(i => i.IsMultiple());
        return moreThan1.SelectMany(i => i);
    }

    public static IEnumerable Duplicates(this IEnumerable source)
    {
        return source.Duplicates(i => i);
    }

    public static bool IsMultiple(this IEnumerable source)
    {
        var enumerator = source.GetEnumerator();
        return enumerator.MoveNext() && enumerator.MoveNext();
    }
}

Using IsMultiple() in the Duplicates method is faster than Count() because this does not iterate the whole collection.