From 9321d394788d5f708cde22006268f45af06d9cfc Mon Sep 17 00:00:00 2001
From: Bond-009 <bond.009@outlook.com>
Date: Sun, 11 Oct 2020 03:21:05 +0200
Subject: [PATCH] Add fast paths to WhenAny(IEnumerable) (#38896)

* Add fast paths to WhenAny(IEnumerable)

Adds fast paths in case `tasks` is an array or an `ICollection`

* Remove cast from slow path for WhenAll and WhenAny
---
 .../src/System/Threading/Tasks/Task.cs             | 31 +++++++++++++++++-----
 1 file changed, 25 insertions(+), 6 deletions(-)

diff --git a/src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/Task.cs b/src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/Task.cs
index 5caf5c0..c6cbb14 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/Task.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/Task.cs
@@ -5469,15 +5469,15 @@ namespace System.Threading.Tasks
         /// </exception>
         public static Task WhenAll(IEnumerable<Task> tasks)
         {
-            // Take a more efficient path if tasks is actually an array
-            if (tasks is Task[] taskArray)
-            {
-                return WhenAll(taskArray);
-            }
-
             // Skip a List allocation/copy if tasks is a collection
             if (tasks is ICollection<Task> taskCollection)
             {
+                // Take a more efficient path if tasks is actually an array
+                if (tasks is Task[] taskArray)
+                {
+                    return WhenAll(taskArray);
+                }
+
                 int index = 0;
                 taskArray = new Task[taskCollection.Count];
                 foreach (Task task in tasks)
@@ -6077,6 +6077,25 @@ namespace System.Threading.Tasks
         /// </exception>
         public static Task<Task> WhenAny(IEnumerable<Task> tasks)
         {
+            // Skip a List allocation/copy if tasks is a collection
+            if (tasks is ICollection<Task> taskCollection)
+            {
+                // Take a more efficient path if tasks is actually an array
+                if (tasks is Task[] taskArray)
+                {
+                    return WhenAny(taskArray);
+                }
+
+                int index = 0;
+                taskArray = new Task[taskCollection.Count];
+                foreach (Task task in tasks)
+                {
+                    if (task == null) ThrowHelper.ThrowArgumentException(ExceptionResource.Task_MultiTaskContinuation_NullTask, ExceptionArgument.tasks);
+                    taskArray[index++] = task;
+                }
+                return TaskFactory.CommonCWAnyLogic(taskArray);
+            }
+
             if (tasks == null) ThrowHelper.ThrowArgumentNullException(ExceptionArgument.tasks);
 
             // Make a defensive copy, as the user may manipulate the tasks collection
-- 
2.7.4