I currently use the following:var result = list.Union(otherList).SelectMany(l => l.children).ToList()foreach(var child in lotsOfChildren){    something = result.Contains(child)}I only use result for querying if the list contains items, using result.Contains(x). This look to be inefficient to me, as, presumably, Contains() has to enumerate the entire (static) list each time, to check if an item is contained. I noticed ToDictionary() and ToLookup() extension methods, however both these require a key to be defined, which is unnecessary.I guess what I'm really looking for is a ToHashSet() sort of thing - would I have to create this manually, as in .ToLookup(c => c.GetHashCode, c => c)?  Or is there a built in method I'm missing? Or, alternatively, is this something the compiler will optimise, and I don't need to care about.
Most efficient LINQ result for Contains() query
c#;efficiency;linq
You can trivially define an extension method for ToHashSet if that's what you're looking for.public static class Extensions{    public static HashSet<T> ToHashSet<T>(this IEnumerable<T> source)    {        return new HashSet<T>(source);    }}You should now be able to use it in your original example:var result = list.Union(otherList).SelectMany(l => l.children).ToHashSet()foreach (var child in lotsOfChildren){    something = result.Contains(child)}