How to convert linq results to HashSet or HashedSet

Question

I have a property on a class that is an ISet. I\'m trying to get the results of a linq query into that property, but can\'t figure out how to do so.

Basically, look

Answer 1

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);

Answer 2

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 :)

Answer 3

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);
}

Answer 4

Jon's answer is perfect. The only caveat is that, using NHibernate's HashedSet, I need to convert the results to a collection. Is there an optimal way to do this?

ISet<string> bla = new HashedSet<string>((from b in strings select b).ToArray()); 

or

ISet<string> bla = new HashedSet<string>((from b in strings select b).ToList()); 

Or am I missing something else?

---

Edit: This is what I ended up doing:

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

public static HashedSet<T> ToHashedSet<T>(this IEnumerable<T> source)
{
    return new HashedSet<T>(source.ToHashSet());
}

Answer 5

Rather than the simple conversion of IEnumerable to a HashSet, it is often convenient to convert a property of another object into a HashSet. You could write this as:

var set = myObject.Select(o => o.Name).ToHashSet();

but, my preference would be to use selectors:

var set = myObject.ToHashSet(o => o.Name);

They do the same thing, and the the second is obviously shorter, but I find the idiom fits my brains better (I think of it as being like ToDictionary).

Here's the extension method to use, with support for custom comparers as a bonus.

public static HashSet<TKey> ToHashSet<TSource, TKey>(
    this IEnumerable<TSource> source,
    Func<TSource, TKey> selector,
    IEqualityComparer<TKey> comparer = null)
{
    return new HashSet<TKey>(source.Select(selector), comparer);
}

Answer 6

This functionality has been added as an extension method on IEnumerable<TSource> to .NET Framework 4.7.2:

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