Imported Upstream version 4.6.0.125

Former-commit-id: a2155e9bd80020e49e72e86c44da02a8ac0e57a4

mcs/class/referencesource/System.Data.Entity/System/Data/Common/Utils/Singleton.cs

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+//---------------------------------------------------------------------
+// <copyright file="Singleton.cs" company="Microsoft">
+//      Copyright (c) Microsoft Corporation.  All rights reserved.
+// </copyright>
+//
+// @owner  	 [....]
+//---------------------------------------------------------------------
+
+using System.Collections.Generic;
+using System.Threading;
+using System.Diagnostics;
+namespace System.Data.Common.Utils
+{
+    /// <summary>
+    /// Allows for delayed creation of a singleton that can be used safely by multiple threads. 
+    /// Instantiation of the singleton instance is not synchronized and may be invoked multiple times by different threads,
+    /// however only one evaluation will 'win' and provide the value for the Singleton instance. 
+    /// The <see cref="Value"/> property is guaranteed to always return the same instance.
+    /// Limitations:
+    /// 1. Reference types only (to simplify the 'is initialized?' check)
+    /// 2. The instantiation function will be invoked whenever the Value property is accessed and the current value is null, 
+    ///    not just the first time; avoid returning null from the instantiation function unless the intent is for instantiation to be retried.
+    /// </summary>
+    /// <typeparam name="TValue">Type of the singleton instance.</typeparam>
+    internal sealed class Singleton<TValue>
+        where TValue : class
+    {
+        private readonly Func<TValue> valueProvider;
+        private TValue value;
+
+        /// <summary>
+        /// Constructs a new Singleton that uses the specified function to instantiate its value.
+        /// </summary>
+        /// <param name="function">Required. Function to evaluate to produce the singleton instance.</param>
+        internal Singleton(Func<TValue> function)
+        {
+            EntityUtil.CheckArgumentNull(function, "function");
+            this.valueProvider = function;
+        }
+
+        /// <summary>
+        /// Retrieves the singleton value, either by evaluating the value function or returning the already cached instance.
+        /// </summary>
+        internal TValue Value
+        {
+            get
+            {
+                TValue result = this.value; // reading of reference types is atomic.
+                if (result == null)
+                {
+                    TValue newValue = this.valueProvider();
+                    Interlocked.CompareExchange(ref this.value, newValue, null);
+                    result = this.value;
+                }
+                return result;
+            }
+        }
+    }
+}