How to convert linq results to HashSet or HashedSet

This functionality has been added as an extension method on IEnumerable<TSource> to .NET Framework 4.7.2 and .NET Core 2.0. It is consequently also available on .NET 5 and later.

• ToHashSet<TSource>(IEnumerable<TSource>)
• ToHashSet<TSource>(IEnumerable<TSource>, IEqualityComparer<TSource>)

I don't think there's anything built in which does this... but it's really easy to write an extension method:

public static class Extensions
{
    public static HashSet<T> ToHashSet<T>(
        this IEnumerable<T> source,
        IEqualityComparer<T> comparer = null)
    {
        return new HashSet<T>(source, comparer);
    }
}

Note that you really do want an extension method (or at least a generic method of some form) here, because you may not be able to express the type of T explicitly:

var query = from i in Enumerable.Range(0, 10)
            select new { i, j = i + 1 };
var resultSet = query.ToHashSet();

You can't do that with an explicit call to the HashSet<T> constructor. We're relying on type inference for generic methods to do it for us.

Now you could choose to name it ToSet and return ISet<T> - but I'd stick with ToHashSet and the concrete type. This is consistent with the standard LINQ operators (ToDictionary, ToList) and allows for future expansion (e.g. ToSortedSet). You may also want to provide an overload specifying the comparison to use.

Just pass your IEnumerable into the constructor for HashSet.

HashSet<T> foo = new HashSet<T>(from x in bar.Items select x);