Imported Upstream version 3.6.0

Former-commit-id: da6be194a6b1221998fc28233f2503bd61dd9d14

external/rx/Rx/NET/Source/System.Reactive.Linq/Reactive/Internal/ConcatSink.cs

@@ -0,0 +1,29 @@
+// Copyright (c) Microsoft Open Technologies, Inc. All rights reserved. See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Generic;
+
+namespace System.Reactive
+{
+    abstract class ConcatSink<TSource> : TailRecursiveSink<TSource>
+    {
+        public ConcatSink(IObserver<TSource> observer, IDisposable cancel)
+                : base(observer, cancel)
+        {
+        }
+
+        protected override IEnumerable<IObservable<TSource>> Extract(IObservable<TSource> source)
+        {
+            var concat = source as IConcatenatable<TSource>;
+            if (concat != null)
+                return concat.GetSources();
+
+            return null;
+        }
+
+        public override void OnCompleted()
+        {
+            _recurse();
+        }
+    }
+}