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