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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.
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原文地址：http://www.cnblogs.com/shouce/p/5335667.html
