.Net 4.0 延迟加载:Lazy<T>

蒋炸役 · 2025-5-29 19:50:58

昨天我们说了一个.Net 4.0里面StringBuilder新增的Clear()方法及其实现,非常简单.今天要说的就稍微复杂一点了.
       语言和模式互相促进,语言让模式实现有更多可能性,越来越多优秀的被语言实现.模式实现的责任从开发者转移到语言.延迟初始化(Lazyinitialization)已经在.Net 4.0中给出了默认实现.本文将探究其使用方法和实现.
  为什么要延迟初始化(Lazy initialization)?
    平时开发能接触到延迟初始化可能是在两个地方,一个是单件模式Singletonpattern,一个是Nhibernate;这两个典型场景很能说明为什么需要延迟初始化:
1.需要初始化的对象属于昂贵的资源,直到使用的时候再初始化load-on-demand
2.初始化过程本身相当复杂,代码中要避免这种无谓复杂性,直到使用再初始化
更多延迟初始化的资料,请点击这里;

.Net 4.0 Lazy实现Lazy initialization
        .Net 4.0中的延迟初始化的默认实现时Lazy,我们通过一个简单的例子看一下怎么使用,为了方便讨论我们新建一个Student的实体类:

public class Student { public int ID { get;set; } public string Name { get; set; } }

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延迟初始化Student:

Lazy<Student> student = newLazy<Student>(); Console.WriteLine(student); student.Value.ID = 23; student.Value.Name = "New"; Console.WriteLine(student);

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我们在Console.WriteLine(student); 一行设置断点查看,发现IsValueCreated是false ,Value值是null,截图如下:

继续往下走,在student.Value.ID = 23;赋值的时候,再次查看student对象的值,IsValueCreated为true,Value已经不为空,见下图.

我们可以猜想,应该是在Value的Get方法中对对象进行了实例化,打开Reflector v6验证:

Lazy

1 [Serializable, DebuggerDisplay("ThreadSafetyMode={Mode}, IsValueCreated={IsValueCreated}, IsValueFaulted={IsValueFaulted}, Value={ValueForDebugDisplay}"), DebuggerTypeProxy(typeof(System_LazyDebugView<>)), ComVisible(false), HostProtection(SecurityAction.LinkDemand, Synchronization=true, ExternalThreading=true)] 2 public class Lazy<T> 3 { 4 // Fields 5 private volatile object m_boxed; 6 [NonSerialized] 7 private readonly object m_threadSafeObj; 8 [NonSerialized] 9 private Func<T> m_valueFactory; 10 private static Func<T> PUBLICATION_ONLY_OR_ALREADY_INITIALIZED; 11 12 // Methods 13 static Lazy() 14 { 15 Lazy<T>.PUBLICATION_ONLY_OR_ALREADY_INITIALIZED = delegate { 16 return default(T); 17 }; 18 } 19 20 [TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")] 21 public Lazy() : this(LazyThreadSafetyMode.ExecutionAndPublication) 22 { 23 } 24 25 public Lazy(bool isThreadSafe) : this(isThreadSafe ? LazyThreadSafetyMode.ExecutionAndPublication : LazyThreadSafetyMode.None) 26 { 27 } 28 29 [TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")] 30 public Lazy(Func<T> valueFactory) : this(valueFactory, LazyThreadSafetyMode.ExecutionAndPublication) 31 { 32 } 33 34 public Lazy(LazyThreadSafetyMode mode) 35 { 36 this.m_threadSafeObj = Lazy<T>.GetObjectFromMode(mode); 37 } 38 39 public Lazy(Func<T> valueFactory, bool isThreadSafe) : this(valueFactory, isThreadSafe ? LazyThreadSafetyMode.ExecutionAndPublication : LazyThreadSafetyMode.None) 40 { 41 } 42 43 public Lazy(Func<T> valueFactory, LazyThreadSafetyMode mode) 44 { 45 if (valueFactory == null) 46 { 47 throw new ArgumentNullException("valueFactory"); 48 } 49 this.m_threadSafeObj = Lazy<T>.GetObjectFromMode(mode); 50 this.m_valueFactory = valueFactory; 51 } 52 53 private Boxed<T> CreateValue() 54 { 55 Boxed<T> boxed = null; 56 LazyThreadSafetyMode mode = this.Mode; 57 if (this.m_valueFactory != null) 58 { 59 try 60 { 61 if ((mode != LazyThreadSafetyMode.PublicationOnly) && (this.m_valueFactory == Lazy<T>.PUBLICATION_ONLY_OR_ALREADY_INITIALIZED)) 62 { 63 throw new InvalidOperationException(Environment.GetResourceString("Lazy_Value_RecursiveCallsToValue")); 64 } 65 Func<T> valueFactory = this.m_valueFactory; 66 if (mode != LazyThreadSafetyMode.PublicationOnly) 67 { 68 this.m_valueFactory = Lazy<T>.PUBLICATION_ONLY_OR_ALREADY_INITIALIZED; 69 } 70 return new Boxed<T>(valueFactory()); 71 } 72 catch (Exception exception) 73 { 74 if (mode != LazyThreadSafetyMode.PublicationOnly) 75 { 76 this.m_boxed = new LazyInternalExceptionHolder<T>(exception.PrepForRemoting()); 77 } 78 throw; 79 } 80 } 81 try 82 { 83 boxed = new Boxed<T>((T) Activator.CreateInstance(typeof(T))); 84 } 85 catch (MissingMethodException) 86 { 87 Exception ex = new MissingMemberException(Environment.GetResourceString("Lazy_CreateValue_NoParameterlessCtorForT")); 88 if (mode != LazyThreadSafetyMode.PublicationOnly) 89 { 90 this.m_boxed = new LazyInternalExceptionHolder<T>(ex); 91 } 92 throw ex; 93 } 94 return boxed; 95 } 96 97 private static object GetObjectFromMode(LazyThreadSafetyMode mode) 98 { 99 if (mode == LazyThreadSafetyMode.ExecutionAndPublication) 100 { 101 return new object(); 102 } 103 if (mode == LazyThreadSafetyMode.PublicationOnly) 104 { 105 return Lazy<T>.PUBLICATION_ONLY_OR_ALREADY_INITIALIZED; 106 } 107 if (mode != LazyThreadSafetyMode.None) 108 { 109 throw new ArgumentOutOfRangeException("mode", Environment.GetResourceString("Lazy_ctor_ModeInvalid")); 110 } 111 return null; 112 } 113 114 private T LazyInitValue() 115 { 116 Boxed<T> boxed = null; 117 switch (this.Mode) 118 { 119 case LazyThreadSafetyMode.None: 120 boxed = this.CreateValue(); 121 this.m_boxed = boxed; 122 break; 123 124 case LazyThreadSafetyMode.PublicationOnly: 125 boxed = this.CreateValue(); 126 if (Interlocked.CompareExchange(ref this.m_boxed, boxed, null) != null) 127 { 128 boxed = (Boxed<T>) this.m_boxed; 129 } 130 break; 131 132 default: 133 { 134 object obj2; 135 bool lockTaken = false; 136 try 137 { 138 Monitor.Enter(obj2 = this.m_threadSafeObj, ref lockTaken); 139 if (this.m_boxed == null) 140 { 141 boxed = this.CreateValue(); 142 this.m_boxed = boxed; 143 } 144 else 145 { 146 boxed = this.m_boxed as Boxed<T>; 147 if (boxed == null) 148 { 149 LazyInternalExceptionHolder<T> holder = this.m_boxed as LazyInternalExceptionHolder<T>; 150 throw holder.m_exception; 151 } 152 } 153 } 154 finally 155 { 156 if (lockTaken) 157 { 158 Monitor.Exit(obj2); 159 } 160 } 161 break; 162 } 163 } 164 return boxed.m_value; 165 } 166 167 [OnSerializing] 168 private void OnSerializing(StreamingContext context) 169 { 170 T local1 = this.Value; 171 } 172 173 public override string ToString() 174 { 175 if (!this.IsValueCreated) 176 { 177 return Environment.GetResourceString("Lazy_ToString_ValueNotCreated"); 178 } 179 return this.Value.ToString(); 180 } 181 182 // Properties 183 public bool IsValueCreated 184 { 185 [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries")] 186 get 187 { 188 return ((this.m_boxed != null) && (this.m_boxed is Boxed<T>)); 189 } 190 } 191 192 internal bool IsValueFaulted 193 { 194 get 195 { 196 return (this.m_boxed is LazyInternalExceptionHolder<T>); 197 } 198 } 199 200 internal LazyThreadSafetyMode Mode 201 { 202 get 203 { 204 if (this.m_threadSafeObj == null) 205 { 206 return LazyThreadSafetyMode.None; 207 } 208 if (this.m_threadSafeObj == Lazy<T>.PUBLICATION_ONLY_OR_ALREADY_INITIALIZED) 209 { 210 return LazyThreadSafetyMode.PublicationOnly; 211 } 212 return LazyThreadSafetyMode.ExecutionAndPublication; 213 } 214 } 215 216 [DebuggerBrowsable(DebuggerBrowsableState.Never)] 217 public T Value 218 { 219 get 220 { 221 Boxed<T> boxed = null; 222 if (this.m_boxed != null) 223 { 224 boxed = this.m_boxed as Boxed<T>; 225 if (boxed != null) 226 { 227 return boxed.m_value; 228 } 229 LazyInternalExceptionHolder<T> holder = this.m_boxed as LazyInternalExceptionHolder<T>; 230 throw holder.m_exception; 231 } 232 Debugger.NotifyOfCrossThreadDependency(); 233 return this.LazyInitValue(); 234 } 235 } 236 237 internal T ValueForDebugDisplay 238 { 239 get 240 { 241 if (!this.IsValueCreated) 242 { 243 return default(T); 244 } 245 return ((Boxed<T>) this.m_boxed).m_value; 246 } 247 } 248 249 // Nested Types 250 [Serializable] 251 private class Boxed 252 { 253 // Fields 254 internal T m_value; 255 256 // Methods 257 [TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")] 258 internal Boxed(T value) 259 { 260 this.m_value = value; 261 } 262 } 263 264 private class LazyInternalExceptionHolder 265 { 266 // Fields 267 internal Exception m_exception; 268 269 // Methods 270 [TargetedPatchingOptOut("Performance critical to inline this type of method across NGen image boundaries")] 271 internal LazyInternalExceptionHolder(Exception ex) 272 { 273 this.m_exception = ex; 274 } 275 } 276 } 277 278

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是不是和我们想的一样呢?
为了更清晰的看出延迟加载的处理逻辑,我按照这个代码的实现思路,去掉复杂应用场景中队异常,多线程等方面的代码,重新实现了一个Lazy,就叫它YaLazy吧

1 public class YaLazy<T> 2 { 3 private bool _isValueCreated = false; 4 public bool IsValueCreated 5 { 6 get 7 { 8 return _isValueCreated; 9 } 10 } 11 private T _value; 12 public T Value 13 { 14 get 15 { 16 if (this._value != null) 17 { 18 return (T)_value; 19 } 20 return CreateValue(); 21 22 } 23 } 24 private T CreateValue() 25 { 26 _isValueCreated = true; 27 _value = (T)Activator.CreateInstance(typeof(T)); 28 return _value; 29 } 30 }

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使用方法和Lazy类似:

1 YaLazy<Student> student2 = new YaLazy<Student>(); 2 Console.WriteLine(student2); 3 student2.Value.ID = 23; 4 student2.Value.Name = "New"; 5 Console.WriteLine(student2); 6

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建议单步调试~

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.Net 4.0 延迟加载:Lazy<T>

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