Adding ToHashSet tests.

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

@@ -0,0 +1,34 @@
+// 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;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using Xunit;
+
+namespace Tests
+{
+    public class ToHashSet : AsyncEnumerableTests
+    {
+        [Fact]
+        public async Task ToHashSet_Null()
+        {
+            await Assert.ThrowsAsync<ArgumentNullException>(() => AsyncEnumerable.ToHashSet<int>(default(IAsyncEnumerable<int>)));
+            await Assert.ThrowsAsync<ArgumentNullException>(() => AsyncEnumerable.ToHashSet<int>(default(IAsyncEnumerable<int>), CancellationToken.None));
+
+            await Assert.ThrowsAsync<ArgumentNullException>(() => AsyncEnumerable.ToHashSet<int>(default(IAsyncEnumerable<int>), EqualityComparer<int>.Default, CancellationToken.None));
+            await Assert.ThrowsAsync<ArgumentNullException>(() => AsyncEnumerable.ToHashSet<int>(AsyncEnumerable.Return(42), default(IEqualityComparer<int>), CancellationToken.None));
+        }
+
+        [Fact]
+        public void ToHashSet1()
+        {
+            var xs = new[] { 1, 2, 1, 2, 3, 4, 1, 2, 3, 4 };
+            var res = xs.ToAsyncEnumerable().ToHashSet();
+            Assert.True(res.Result.OrderBy(x => x).SequenceEqual(new[] { 1, 2, 3, 4 }));
+        }
+    }
+}