延迟加载主要应用在以下场景:
数据层 – ADO.NET或Entity Framework等ORM
反射 – 加载assemblies, types, MEF
缓存对象,领域实体
下面以领域实体为例, 在实体中我们经常检查某个属性下字段是否为空引用. 如果是空的话,计算或填充这个字段. 像这样:
1: /// <summary> 2: /// Order 3: /// </summary> 4: public class Order 5: { 6: private Customer _customer; 7: /// <summary> 8: /// Gets the customer. 9: /// </summary> 10: public Customer Customer 11: { 12: get 13: { 14: if (_customer == null) 15: { 16: _customer = new Customer(); 17: } return _customer; 18: } 19: } 20: /// <summary> 21: /// Prints the label. 22: /// </summary> 23: /// <returns></returns> 24: public string PrintLabel() 25: { 26: return Customer.CompanyName + "\n" + Customer.Address; 27: } 28: }如果PrintLabel方法写成这样:
1: public string PrintLabel() 2: { 3: string result = _customer.CompanyName; // probably results in a NullReferenceException 4: return result + "\n" + Customer.Address; // ok to access Customer 5: }注意上面第4行代码,可能有空引用异常. 下面我们使用Lazy<T>来解决这个问题:
1: /// <summary> 2: /// Order 3: /// </summary> 4: public class Order 5: { 6: /// <summary> 7: /// _customerInitializer 8: /// </summary> 9: private Lazy<Customer> _customerInitializer; 10: 11: /// <summary> 12: /// Initializes a new instance of the <see cref="Order"/> class. 13: /// </summary> 14: public Order() 15: { 16: _customerInitializer = new Lazy<Customer>(() => new Customer(),isThreadSafe:true); 17: } 18: 19: /// <summary> 20: /// Gets the customer. 21: /// </summary> 22: public Customer Customer 23: { 24: get{ return _customerInitializer.Value;} 25: } 26: 27: /// <summary> 28: /// Prints the label. 29: /// </summary> 30: /// <returns></returns> 31: public string PrintLabel() 32: { 33: string result = Customer.CompanyName; // ok to access Customer 34: return result + "\n" + _customerInitializer.Value.Address; // ok to access via .Value 35: } 36: }上面的代码16行,我们延迟初始化对像实例,并且设置线程安全. Lazy<T>内部是使用Func<T>来实现的,看下面其中构造器的源代码:
1: public Lazy(Func<T> valueFactory, LazyThreadSafetyMode mode) 2: { 3: if (valueFactory == null) throw new ArgumentNullException("valueFactory"); 4: this.m_threadSafeObj = Lazy<T>.GetObjectFromMode(mode); 5: this.m_valueFactory = valueFactory; 6: }有兴趣请自行查看.net framework中的源码. 我们还可以扩展一个LazyNotNull<T>
1: /// <summary> 2: /// LazyNotNull 3: /// </summary> 4: /// <typeparam name="T">Type</typeparam> 5: public class LazyNotNull<T> 6: { 7: private Lazy<T> _lazyValue = null; 8: private Func<T> _valueFactory = null; 9: 10: /// <summary> 11: /// Initializes a new instance of the <see cref="LazyNotNull<T>"/> class. 12: /// </summary> 13: /// <param name="valueFactory">The value factory.</param> 14: public LazyNotNull(Func<T> valueFactory) 15: { 16: _lazyValue = new Lazy<T>(valueFactory); 17: _valueFactory = valueFactory; 18: } 19: 20: /// <summary> 21: /// Gets T value. 22: /// </summary> 23: public T Value 24: { 25: get 26: { 27: var lazyValue = _lazyValue; 28: if (lazyValue.Value != null) 29: { 30: return lazyValue.Value; 31: } 32: _lazyValue = new Lazy<T>(_valueFactory); 33: return default(T); 34: } 35: } 36: }
希望这篇POST对您开发有帮助.