[core] Take advantage of IThreadPoolWorkItem

This can be used to provide an allocation-free way to enqueue
tasks in the threadpool.

Author: alanmcgovern

src/ReusableTasks/ActionWorkItem.cs

@@ -0,0 +1,43 @@
+using System;
+using System.Collections.Generic;
+using System.Threading;
+
+namespace ReusableTasks
+{
+#if NET5_0_OR_GREATER || NETCOREAPP3_0_OR_GREATER
+    class ActionWorkItem : IThreadPoolWorkItem
+    {
+        static readonly Action EmptyAction = () => { };
+        static readonly Stack<ActionWorkItem> Cache = new Stack<ActionWorkItem>();
+
+        public Action Continuation { get; private set; } = EmptyAction;
+
+        public static ActionWorkItem GetOrCreate(Action action)
+        {
+            lock (Cache)
+            {
+                if (Cache.Count == 0)
+                {
+                    return new ActionWorkItem { Continuation = action };
+                }
+                else
+                {
+                    var worker = Cache.Pop();
+                    worker.Continuation = action;
+                    return worker;
+                }
+            }
+        }
+
+        public void Execute()
+        {
+            var continuation = Continuation;
+            Continuation = EmptyAction;
+
+            lock (Cache)
+                Cache.Push(this);
+            continuation();
+        }
+    }
+#endif
+}

src/ReusableTasks/ReusableTasks.csproj

@@ -2,7 +2,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>netstandard2.0;netstandard2.1</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0;netstandard2.1;net5.0;netcoreapp3.0</TargetFrameworks>
 
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
 

src/ReusableTasks/System.Runtime.CompilerServices/ResultHolder_T.cs

@@ -205,7 +205,11 @@ void TryInvoke (Action callback)
             else if (SyncContext != null)
                 SyncContext.Post (InvokeOnContext, callback);
             else
+#if NET5_0_OR_GREATER || NETCOREAPP3_0_OR_GREATER
+                ThreadPool.UnsafeQueueUserWorkItem (ActionWorkItem.GetOrCreate (callback), false);
+#else
                 ThreadPool.UnsafeQueueUserWorkItem (InvokeOnThreadPool, callback);
+#endif
         }
     }
 }