Imported Upstream version 4.6.0.125

Former-commit-id: a2155e9bd80020e49e72e86c44da02a8ac0e57a4

mcs/class/referencesource/System.Web/UI/PageAsyncTaskApm.cs

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+//------------------------------------------------------------------------------
+// <copyright file="PageAsyncTaskApm.cs" company="Microsoft">
+//     Copyright (c) Microsoft Corporation.  All rights reserved.
+// </copyright>
+//------------------------------------------------------------------------------
+
+namespace System.Web.UI {
+    using System;
+    using System.Threading;
+    using System.Threading.Tasks;
+
+    // Represents an IPageAsyncTask that follows APM (Begin / End)
+
+    internal sealed class PageAsyncTaskApm : IPageAsyncTask {
+
+        private readonly BeginEventHandler _beginHandler;
+        private readonly EndEventHandler _endHandler;
+        private readonly object _state;
+
+        public PageAsyncTaskApm(BeginEventHandler beginHandler, EndEventHandler endHandler, object state) {
+            _beginHandler = beginHandler;
+            _endHandler = endHandler;
+            _state = state;
+        }
+
+        public async Task ExecuteAsync(object sender, EventArgs e, CancellationToken cancellationToken) {
+            // The CancellationToken is ignored in APM since every call to Begin must be matched by
+            // a call to End, otherwise memory leaks and other badness can occur.
+
+            // The reason we don't use TaskFactory.FromAsync is that we need the end handler to execute
+            // within the current synchronization context, but FromAsync doesn't make that guarantee.
+
+            // The callback that marks the TaskCompletionSource as complete can execute synchronously or asynchronously.
+            TaskCompletionSource<object> taskCompletionSource = new TaskCompletionSource<object>();
+            IAsyncResult asyncResult = _beginHandler(sender, e, _ => { taskCompletionSource.SetResult(null); }, _state);
+
+            await taskCompletionSource.Task;
+            _endHandler(asyncResult);
+        }
+
+    }
+}