Several updates to ValueTask

src/mscorlib/shared/System/Runtime/CompilerServices/ConfiguredValueTaskAwaitable.cs

@@ -35,10 +35,10 @@ public ConfiguredValueTaskAwaiter GetAwaiter() =>
 
         /// <summary>Provides an awaiter for a <see cref="ConfiguredValueTaskAwaitable{TResult}"/>.</summary>
         [StructLayout(LayoutKind.Auto)]
-        public struct ConfiguredValueTaskAwaiter : ICriticalNotifyCompletion, IConfiguredValueTaskAwaiter
+        public readonly struct ConfiguredValueTaskAwaiter : ICriticalNotifyCompletion, IConfiguredValueTaskAwaiter
         {
             /// <summary>The value being awaited.</summary>
-            private ValueTask<TResult> _value; // Methods are called on this; avoid making it readonly so as to avoid unnecessary copies
+            private readonly ValueTask<TResult> _value;
             /// <summary>The value to pass to ConfigureAwait.</summary>
             internal readonly bool _continueOnCapturedContext;
 
@@ -74,7 +74,11 @@ public void UnsafeOnCompleted(Action continuation) =>
 
             /// <summary>Gets the task underlying the incomplete <see cref="_value"/>.</summary>
             /// <remarks>This method is used when awaiting and IsCompleted returned false; thus we expect the value task to be wrapping a non-null task.</remarks>
-            (Task task, bool continueOnCapturedContext) IConfiguredValueTaskAwaiter.GetTask() => (_value.AsTaskExpectNonNull(), _continueOnCapturedContext);
+            Task IConfiguredValueTaskAwaiter.GetTask(out bool continueOnCapturedContext)
+            {
+                continueOnCapturedContext = _continueOnCapturedContext;
+                return _value.AsTaskExpectNonNull();
+            }
         }
     }
 
@@ -83,6 +87,6 @@ public void UnsafeOnCompleted(Action continuation) =>
     /// </summary>
     internal interface IConfiguredValueTaskAwaiter
     {
-        (Task task, bool continueOnCapturedContext) GetTask();
+        Task GetTask(out bool continueOnCapturedContext);
     }
 }

src/mscorlib/shared/System/Runtime/CompilerServices/ValueTaskAwaiter.cs

@@ -8,10 +8,10 @@
 namespace System.Runtime.CompilerServices
 {
     /// <summary>Provides an awaiter for a <see cref="ValueTask{TResult}"/>.</summary>
-    public struct ValueTaskAwaiter<TResult> : ICriticalNotifyCompletion, IValueTaskAwaiter
+    public readonly struct ValueTaskAwaiter<TResult> : ICriticalNotifyCompletion, IValueTaskAwaiter
     {
         /// <summary>The value being awaited.</summary>
-        private ValueTask<TResult> _value; // Methods are called on this; avoid making it readonly so as to avoid unnecessary copies
+        private readonly ValueTask<TResult> _value;
 
         /// <summary>Initializes the awaiter.</summary>
         /// <param name="value">The value to be awaited.</param>

src/mscorlib/shared/System/Threading/Tasks/ValueTask.cs

@@ -3,7 +3,6 @@
 // See the LICENSE file in the project root for more information.
 
 using System.Collections.Generic;
-using System.ComponentModel;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 
@@ -77,7 +76,7 @@ public ValueTask(Task<TResult> task)
             }
 
             _task = task;
-            _result = default(TResult);
+            _result = default;
         }
 
         /// <summary>Returns the hash code for this instance.</summary>
@@ -114,7 +113,12 @@ public Task<TResult> AsTask() =>
             // Return the task if we were constructed from one, otherwise manufacture one.  We don't
             // cache the generated task into _task as it would end up changing both equality comparison
             // and the hash code we generate in GetHashCode.
-            _task ?? AsyncTaskMethodBuilder<TResult>.GetTaskForResult(_result);
+            _task ??
+#if netstandard
+                Task.FromResult(_result);
+#else
+                AsyncTaskMethodBuilder<TResult>.GetTaskForResult(_result);
+#endif
 
         internal Task<TResult> AsTaskExpectNonNull() =>
             // Return the task if we were constructed from one, otherwise manufacture one.
@@ -123,13 +127,24 @@ internal Task<TResult> AsTaskExpectNonNull() =>
             _task ?? GetTaskForResultNoInlining();
 
         [MethodImpl(MethodImplOptions.NoInlining)]
-        private Task<TResult> GetTaskForResultNoInlining() => AsyncTaskMethodBuilder<TResult>.GetTaskForResult(_result);
+        private Task<TResult> GetTaskForResultNoInlining() =>
+#if netstandard
+            Task.FromResult(_result);
+#else
+            AsyncTaskMethodBuilder<TResult>.GetTaskForResult(_result);
+#endif
 
         /// <summary>Gets whether the <see cref="ValueTask{TResult}"/> represents a completed operation.</summary>
         public bool IsCompleted => _task == null || _task.IsCompleted;
 
         /// <summary>Gets whether the <see cref="ValueTask{TResult}"/> represents a successfully completed operation.</summary>
-        public bool IsCompletedSuccessfully => _task == null || _task.IsCompletedSuccessfully;
+        public bool IsCompletedSuccessfully =>
+            _task == null ||
+#if netstandard
+            _task.Status == TaskStatus.RanToCompletion;
+#else
+            _task.IsCompletedSuccessfully;
+#endif
 
         /// <summary>Gets whether the <see cref="ValueTask{TResult}"/> represents a failed operation.</summary>
         public bool IsFaulted => _task != null && _task.IsFaulted;
@@ -153,26 +168,16 @@ public ConfiguredValueTaskAwaitable<TResult> ConfigureAwait(bool continueOnCaptu
         /// <summary>Gets a string-representation of this <see cref="ValueTask{TResult}"/>.</summary>
         public override string ToString()
         {
-            if (_task != null)
+            if (IsCompletedSuccessfully)
             {
-                return _task.IsCompletedSuccessfully && _task.Result != null ?
-                    _task.Result.ToString() :
-                    string.Empty;
+                TResult result = Result;
+                if (result != null)
+                {
+                    return result.ToString();
+                }
             }
-            else
-            {
-                return _result != null ?
-                    _result.ToString() :
-                    string.Empty;
-            }
-        }
 
-        // TODO https://github.com/dotnet/corefx/issues/22171:
-        // Remove CreateAsyncMethodBuilder once the C# compiler relies on the AsyncBuilder attribute.
-
-        /// <summary>Creates a method builder for use with an async method.</summary>
-        /// <returns>The created builder.</returns>
-        [EditorBrowsable(EditorBrowsableState.Never)] // intended only for compiler consumption
-        public static AsyncValueTaskMethodBuilder<TResult> CreateAsyncMethodBuilder() => AsyncValueTaskMethodBuilder<TResult>.Create();
+            return string.Empty;
+        }
     }
 }

src/mscorlib/src/System/Runtime/CompilerServices/AsyncMethodBuilder.cs

@@ -413,8 +413,8 @@ public void AwaitUnsafeOnCompleted<TAwaiter, TStateMachine>(
             }
             else if ((null != (object)default(TAwaiter)) && (awaiter is IConfiguredValueTaskAwaiter))
             {
-                (Task task, bool continueOnCapturedContext) t = ((IConfiguredValueTaskAwaiter)awaiter).GetTask();
-                TaskAwaiter.UnsafeOnCompletedInternal(t.task, box, t.continueOnCapturedContext);
+                Task t = ((IConfiguredValueTaskAwaiter)awaiter).GetTask(out bool continueOnCapturedContext);
+                TaskAwaiter.UnsafeOnCompletedInternal(t, box, continueOnCapturedContext);
             }
             // The awaiter isn't specially known. Fall back to doing a normal await.
             else