Adding Append.

AsyncRx.NET/System.Reactive.Async/System/Reactive/Linq/Operators/Append.cs

@@ -0,0 +1,36 @@
+// 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. 
+
+namespace System.Reactive.Linq
+{
+    partial class AsyncObservable
+    {
+        public static IAsyncObservable<TSource> Append<TSource>(this IAsyncObservable<TSource> source, TSource element)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+
+            return Create<TSource>(observer => source.SubscribeAsync(AsyncObserver.Append(observer, element)));
+        }
+    }
+
+    partial class AsyncObserver
+    {
+        public static IAsyncObserver<TSource> Append<TSource>(IAsyncObserver<TSource> observer, TSource element)
+        {
+            if (observer == null)
+                throw new ArgumentNullException(nameof(observer));
+
+            return Create<TSource>(
+                observer.OnNextAsync,
+                observer.OnErrorAsync,
+                async () =>
+                {
+                    await observer.OnNextAsync(element).ConfigureAwait(false);
+                    await observer.OnCompletedAsync().ConfigureAwait(false);
+                }
+            );
+        }
+    }
+}