How to convert linq results to HashSet or HashedSet

Question

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);

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>)

As @Joel stated, you can just pass your enumerable in. If you want to do an extension method, you can do:

public static HashSet<T> ToHashSet<T>(this IEnumerable<T> items)
{
    return new HashSet<T>(items);
}

There is an extension method build in the .NET framework and in .NET core for converting an IEnumerable to a HashSet: https://docs.microsoft.com/en-us/dotnet/api/?term=ToHashSet

public static System.Collections.Generic.HashSet<TSource> ToHashSet<TSource> (this System.Collections.Generic.IEnumerable<TSource> source);

It appears that I cannot use it in .NET standard libraries yet (at the time of writing). So then I use this extension method:

    [Obsolete("In the .NET framework and in NET core this method is available, " +
              "however can't use it in .NET standard yet. When it's added, please remove this method")]
public static HashSet<T> ToHashSet<T>(this IEnumerable<T> source, IEqualityComparer<T> comparer = null) => new HashSet<T>(source, comparer);

If you need just readonly access to the set and the source is a parameter to your method, then I would go with

public static ISet<T> EnsureSet<T>(this IEnumerable<T> source)
{
    ISet<T> result = source as ISet<T>;
    if (result != null)
        return result;
    return new HashSet<T>(source);
}

The reason is, that the users may call your method with the ISet already so you do not need to create the copy.

That's pretty simple :)

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

and yes T is the type specified by OP :)