Adding For.

AsyncRx.NET/System.Reactive.Async.Core/System/Reactive/AsyncObservable.cs

@@ -26,4 +26,4 @@ namespace System.Reactive
             return _subscribeAsync(observer);
         }
     }
-}
+}

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

@@ -0,0 +1,93 @@
+// 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.Reactive.Disposables;
+using System.Threading.Tasks;
+
+namespace System.Reactive.Linq
+{
+    partial class AsyncObservable
+    {
+        // REVIEW: Use a tail-recursive sink.
+        // TODO: Add IAsyncEnumerable<T> based overlaod.
+
+        public static IAsyncObservable<TResult> For<TSource, TResult>(IEnumerable<TSource> source, Func<TSource, IAsyncObservable<TResult>> resultSelector)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+            if (resultSelector == null)
+                throw new ArgumentNullException(nameof(resultSelector));
+
+            return For(source, x => Task.FromResult(resultSelector(x)));
+        }
+
+        public static IAsyncObservable<TResult> For<TSource, TResult>(IEnumerable<TSource> source, Func<TSource, Task<IAsyncObservable<TResult>>> resultSelector)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+            if (resultSelector == null)
+                throw new ArgumentNullException(nameof(resultSelector));
+
+            return Create<TResult>(async observer =>
+            {
+                var subscription = new SerialAsyncDisposable();
+
+                var enumerator = source.GetEnumerator();
+
+                var o = default(IAsyncObserver<TResult>);
+
+                o = AsyncObserver.CreateUnsafe<TResult>(
+                        observer.OnNextAsync,
+                        observer.OnErrorAsync,
+                        MoveNext
+                    );
+
+                async Task MoveNext()
+                {
+                    var b = default(bool);
+                    var next = default(IAsyncObservable<TResult>);
+
+                    try
+                    {
+                        b = enumerator.MoveNext();
+
+                        if (b)
+                        {
+                            next = await resultSelector(enumerator.Current).ConfigureAwait(false);
+                        }
+                    }
+                    catch (Exception ex)
+                    {
+                        await observer.OnErrorAsync(ex).ConfigureAwait(false);
+                        return;
+                    }
+
+                    if (b)
+                    {
+                        var sad = new SingleAssignmentAsyncDisposable();
+                        await subscription.AssignAsync(sad).ConfigureAwait(false);
+
+                        var d = await next.SubscribeSafeAsync(o).ConfigureAwait(false);
+                        await sad.AssignAsync(d).ConfigureAwait(false);
+                    }
+                    else
+                    {
+                        await observer.OnCompletedAsync().ConfigureAwait(false);
+                    }
+                }
+
+                await MoveNext().ConfigureAwait(false);
+
+                var disposeEnumerator = AsyncDisposable.Create(() =>
+                {
+                    enumerator.Dispose();
+                    return Task.CompletedTask;
+                });
+
+                return StableCompositeAsyncDisposable.Create(disposeEnumerator, subscription);
+            });
+        }
+    }
+}