Adding SingleAssignmentAsyncDisposable.

AsyncRx.NET/System.Reactive.Async/System/Reactive/Disposables/SingleAssignmentAsyncDisposable.cs

@@ -0,0 +1,37 @@
+// 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.Threading;
+using System.Threading.Tasks;
+
+namespace System.Reactive.Disposables
+{
+    public sealed class SingleAssignmentAsyncDisposable : IAsyncDisposable
+    {
+        private static readonly IAsyncDisposable Disposed = AsyncDisposable.Create(() => Task.CompletedTask);
+
+        private IAsyncDisposable disposable;
+
+        public async Task AssignAsync(IAsyncDisposable disposable)
+        {
+            if (disposable == null)
+                throw new ArgumentNullException(nameof(disposable));
+
+            var old = Interlocked.CompareExchange(ref this.disposable, disposable, null);
+
+            if (old == null)
+                return;
+
+            if (old != Disposed)
+                throw new InvalidOperationException("Disposable already assigned.");
+
+            await disposable.DisposeAsync().ConfigureAwait(false);
+        }
+
+        public Task DisposeAsync()
+        {
+            return Interlocked.Exchange(ref disposable, Disposed)?.DisposeAsync() ?? Task.CompletedTask;
+        }
+    }
+}