我需要一个单身人士：
>是懒惰加载
>是线程安全的
>在施工时加载一些值
>可以随时查询这些值
>初始化可能在查询开始之前的某个精确时间发生 – 所以我必须能够以某种方式从外部触发它.当然,多次触发应该只进行一次初始化.
我使用.NET 3.5.
我使用静态子类开始使用Jon Skeet的implementation(第5版)：
public sealed class Singleton
{
IEnumerable<string> Values {get; private set;}
private Singleton()
{
Values = new[]{"quick","brown","fox"};
}
public static Singleton Instance { get { return Nested.instance; } }
private class Nested
{
// Explicit static constructor to tell C# compiler
// not to mark type as beforefieldinit
static Nested()
{
}
internal static readonly Singleton instance = new Singleton();
}
}
除了“从外部触发初始化”之外,几乎所有方框都会打勾.由于实际初始化发生在ctor内部,因此不会发生多次.
如何实现这一目标？
单例将使用如下：
public static void Main(){
//do stuff,singleton should not yet be initialized.
//the time comes to initialize the singleton,e.g. a database connection is available
//this may be called 0 or more times,possibly on different threads
Singleton.Initialize();
Singleton.Initialize();
Singleton.Initialize();
//actual call to get retrieved values,should work
var retrieveVals = Singleton.Instance.Values;
}
解决方法
好像你可以这样做：
public sealed class Singleton
{
IEnumerable<string> Values {get; private set;}
private Singleton(bool loadDefaults)
{
if (loadDefaults)
Values = new[]{"quick","fox"};
else
Values = new[]{"another","set","of","values"};
}
public static Singleton Instance { get { return Nested.instance; } }
public static void Initialize() {
Nested.Initialize();
}
private class Nested
{
// Explicit static constructor to tell C# compiler
// not to mark type as beforefieldinit
static Nested()
{
}
internal static readonly Singleton instance = new Singleton(true);
private static object instanceLock = new object();
private static bool isInitialized = false;
public static void Initialize() {
lock(instanceLock) {
if (!isInitialized) {
isInitialized = true;
instance = new Singleton(false);
}
}
}
}
}
或者创建一个将要更新的单个实例：
public sealed class Singleton
{
IEnumerable<string> Values {get; private set;}
private Singleton()
{
Values = new[]{"quick","fox"};
}
public static Singleton Instance { get { return Nested.instance; } }
private static object instanceLock = new object();
private static bool isInitialized = false;
public static void Initialize() {
lock(instanceLock) {
if (!isInitialized) {
isInitialized = true;
Instance.Values = new[]{"another","values"};
}
}
}
private class Nested
{
// Explicit static constructor to tell C# compiler
// not to mark type as beforefieldinit
static Nested()
{
}
internal static readonly Singleton instance = new Singleton();
}
}
第三个变体基于您的不可变注释和删除嵌套类注释：
public sealed class Singleton
{
IEnumerable<string> Values {get; private set;}
private Singleton()
{
Values = new[]{"quick","fox"};
}
private static Singleton instance;
private static object instanceLock = new object();
public static Singleton Instance {
get {
Initialize();
return instance;
}
}
public static void Initialize() {
if (instance == null) {
lock(instanceLock) {
if (instance == null)
instance = new Singleton();
}
}
}
}
