Fix SelectMany bug

2 files changed, 40 insertions, 1 deletions

diff --git a/src/System.Linq/src/System/Linq/SelectMany.cs b/src/System.Linq/src/System/Linq/SelectMany.cs
index 5a4b81abf3..d3dc30f321 100644
--- a/src/System.Linq/src/System/Linq/SelectMany.cs
+++ b/src/System.Linq/src/System/Linq/SelectMany.cs
@@ -242,7 +242,7 @@ namespace System.Linq
                         continue;
                     }
 
-                    builder.AddRange(_selector(element));
+                    builder.AddRange(enumerable);
                 }
 
                 TResult[] array = builder.ToArray();
diff --git a/src/System.Linq/tests/SelectManyTests.cs b/src/System.Linq/tests/SelectManyTests.cs
index ebed4cd158..637b8a5448 100644
--- a/src/System.Linq/tests/SelectManyTests.cs
+++ b/src/System.Linq/tests/SelectManyTests.cs
@@ -475,5 +475,44 @@ namespace System.Linq.Tests
             var iterator = counts.SelectMany(c => Enumerable.Range(1, c));
             Assert.Throws<OverflowException>(() => iterator.Count());
         }
+
+        [Theory]
+        [InlineData(10)]
+        public void EvaluateSelectorOncePerItem(int count)
+        {
+            int[] timesCalledMap = new int[count];
+
+            IEnumerable<int> source = Enumerable.Range(0, 10);
+            IEnumerable<int> iterator = source.SelectMany(index =>
+            {
+                timesCalledMap[index]++;
+                return new[] { index };
+            });
+            
+            // Iteration
+            foreach (int index in iterator)
+            {
+                Assert.Equal(Enumerable.Repeat(1, index + 1), timesCalledMap.Take(index + 1));
+                Assert.Equal(Enumerable.Repeat(0, timesCalledMap.Length - index - 1), timesCalledMap.Skip(index + 1));
+            }
+
+            Array.Clear(timesCalledMap, 0, timesCalledMap.Length);
+
+            // ToArray
+            iterator.ToArray();
+            Assert.Equal(Enumerable.Repeat(1, timesCalledMap.Length), timesCalledMap);
+
+            Array.Clear(timesCalledMap, 0, timesCalledMap.Length);
+
+            // ToList
+            iterator.ToList();
+            Assert.Equal(Enumerable.Repeat(1, timesCalledMap.Length), timesCalledMap);
+
+            Array.Clear(timesCalledMap, 0, timesCalledMap.Length);
+
+            // ToHashSet
+            iterator.ToHashSet();
+            Assert.Equal(Enumerable.Repeat(1, timesCalledMap.Length), timesCalledMap);
+        }
     }
 }