Adding ToHashSet.

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/ToHashSet.cs

@@ -0,0 +1,57 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the Apache 2.0 License.
+// See the LICENSE file in the project root for more information. 
+
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace System.Linq
+{
+    public static partial class AsyncEnumerable
+    {
+        public static Task<HashSet<TSource>> ToHashSet<TSource>(this IAsyncEnumerable<TSource> source)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+
+            return ToHashSet(source, EqualityComparer<TSource>.Default, CancellationToken.None);
+        }
+
+        public static Task<HashSet<TSource>> ToHashSet<TSource>(this IAsyncEnumerable<TSource> source, CancellationToken cancellationToken)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+
+            return ToHashSet(source, EqualityComparer<TSource>.Default, cancellationToken);
+        }
+
+        public static Task<HashSet<TSource>> ToHashSet<TSource>(this IAsyncEnumerable<TSource> source, IEqualityComparer<TSource> comparer)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+            if (comparer == null)
+                throw new ArgumentNullException(nameof(comparer));
+
+            return ToHashSet(source, comparer, CancellationToken.None);
+        }
+
+        public static Task<HashSet<TSource>> ToHashSet<TSource>(this IAsyncEnumerable<TSource> source, IEqualityComparer<TSource> comparer, CancellationToken cancellationToken)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+            if (comparer == null)
+                throw new ArgumentNullException(nameof(comparer));
+
+            return source.Aggregate(
+                new HashSet<TSource>(comparer),
+                (set, x) =>
+                {
+                    set.Add(x);
+                    return set;
+                },
+                cancellationToken
+            );
+        }
+    }
+}