Adding ElementAt[OrDefault].

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

@@ -0,0 +1,48 @@
+// 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.Tasks;
+
+namespace System.Reactive.Linq
+{
+    partial class AsyncObservable
+    {
+        public static IAsyncObservable<TSource> ElementAt<TSource>(this IAsyncObservable<TSource> source, int index)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+            if (index < 0)
+                throw new ArgumentOutOfRangeException(nameof(index));
+
+            return Create<TSource>(observer => source.SubscribeAsync(AsyncObserver.ElementAt(observer, index)));
+        }
+    }
+
+    partial class AsyncObserver
+    {
+        public static IAsyncObserver<TSource> ElementAt<TSource>(IAsyncObserver<TSource> observer, int index)
+        {
+            if (observer == null)
+                throw new ArgumentNullException(nameof(observer));
+            if (index < 0)
+                throw new ArgumentOutOfRangeException(nameof(index));
+
+            return Create<TSource>(
+                async x =>
+                {
+                    if (index-- == 0)
+                    {
+                        await observer.OnNextAsync(x).ConfigureAwait(false);
+                        await observer.OnCompletedAsync().ConfigureAwait(false);
+                    }
+                },
+                observer.OnErrorAsync,
+                async () =>
+                {
+                    await observer.OnErrorAsync(new ArgumentOutOfRangeException("The element at the specified index was not found.")).ConfigureAwait(false);
+                }
+            );
+        }
+    }
+}

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

@@ -0,0 +1,47 @@
+// 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> ElementAtOrDefault<TSource>(this IAsyncObservable<TSource> source, int index)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+            if (index < 0)
+                throw new ArgumentOutOfRangeException(nameof(index));
+
+            return Create<TSource>(observer => source.SubscribeAsync(AsyncObserver.ElementAtOrDefault(observer, index)));
+        }
+    }
+
+    partial class AsyncObserver
+    {
+        public static IAsyncObserver<TSource> ElementAtOrDefault<TSource>(IAsyncObserver<TSource> observer, int index)
+        {
+            if (observer == null)
+                throw new ArgumentNullException(nameof(observer));
+            if (index < 0)
+                throw new ArgumentOutOfRangeException(nameof(index));
+
+            return Create<TSource>(
+                async x =>
+                {
+                    if (index-- == 0)
+                    {
+                        await observer.OnNextAsync(x).ConfigureAwait(false);
+                        await observer.OnCompletedAsync().ConfigureAwait(false);
+                    }
+                },
+                observer.OnErrorAsync,
+                async () =>
+                {
+                    await observer.OnNextAsync(default(TSource)).ConfigureAwait(false);
+                    await observer.OnCompletedAsync().ConfigureAwait(false);
+                }
+            );
+        }
+    }
+}