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//---------------------------------------------------------------------
// <copyright file="Singleton.cs" company="Microsoft">
// Copyright (c) Microsoft Corporation. All rights reserved.
// </copyright>
//
// @owner Microsoft
//---------------------------------------------------------------------
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;
}
}
}
}
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