Ix: Improve Defer() by removing indirection

Ix.NET/Source/Benchmarks.System.Interactive/DeferBenchmark.cs

@@ -0,0 +1,27 @@
+// 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;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+using BenchmarkDotNet.Attributes;
+
+namespace Benchmarks.System.Interactive
+{
+    [MemoryDiagnoser]
+    public class DeferBenchmark
+    {
+        [Params(1, 10, 100, 1000, 10000, 100000, 1000000)]
+        public int N;
+        private int _store;
+
+        [Benchmark]
+        public void Defer()
+        {
+            EnumerableEx.Defer(() => Enumerable.Range(1, N))
+                .Subscribe(v => Volatile.Write(ref _store, v));
+        }
+    }
+}

Ix.NET/Source/Benchmarks.System.Interactive/Program.cs

@@ -18,6 +18,7 @@ namespace Benchmarks.System.Interactive
 
             var switcher = new BenchmarkSwitcher(new[] {
                 typeof(BufferCountBenchmark),
+                typeof(DeferBenchmark),
             });
 
             switcher.Run();

Ix.NET/Source/System.Interactive/Defer.cs

@@ -2,6 +2,7 @@
 // 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;
 using System.Collections.Generic;
 
 namespace System.Linq
@@ -26,10 +27,33 @@ namespace System.Linq
 
         private static IEnumerable<TSource> Defer_<TSource>(Func<IEnumerable<TSource>> enumerableFactory)
         {
+            /*
             foreach (var item in enumerableFactory())
             {
                 yield return item;
             }
+            */
+            return new DeferEnumerable<TSource>(enumerableFactory);
+        }
+
+        private sealed class DeferEnumerable<TSource> : IEnumerable<TSource>
+        {
+            readonly Func<IEnumerable<TSource>> _enumerableFactory;
+
+            public DeferEnumerable(Func<IEnumerable<TSource>> enumerableFactory)
+            {
+                _enumerableFactory = enumerableFactory;
+            }
+
+            public IEnumerator<TSource> GetEnumerator()
+            {
+                return _enumerableFactory().GetEnumerator();
+            }
+
+            IEnumerator IEnumerable.GetEnumerator()
+            {
+                return GetEnumerator();
+            }
         }
     }
 }