在软件开发中，延迟加载是一种常见的优化技术，它允许程序在需要时才加载数据，从而提高应用程序的性能和响应速度。本文将通过一个具体的例子，探讨如何在C#中实现数据实体的延迟加载功能。
假设有一个用户(User)实体，它包含用户类型(UserType)的属性。在不使用延迟加载的情况下，User实体的UserType属性会立即加载。但是，如果使用延迟加载，那么UserType属性只有在实际需要时才会加载。
以下是非延迟加载的User和UserType类的简单定义：
public class User {
public int id;
public UserType userType;
}
public class UserType {
public int id;
public string Name;
}
希望实现以下功能：
以下是实现延迟加载功能的User类的代码：
public class User {
public int Id { get; set; }
public string Name { get; set; }
private LazyProperty _userType = new LazyProperty((id) => new UserType(id), (u) => u.Id);
public int? UserTypeId {
get { return _userType.Id; }
set { _userType.Set(value); }
}
public UserType UserType {
get { return _userType.Value; }
set { _userType.Set(value); }
}
}
为了实现上述功能，需要定义一个LazyProperty类。以下是LazyProperty类的代码：
using System;
using System.Threading;
public class LazyProperty where T : class {
public delegate T CreateProperty(int id);
public delegate int GetId(T item);
int? _id = null;
bool? _isThreadSafe;
T _value;
Lazy _Lazy;
GetId _GetId;
CreateProperty _createEntity;
LazyThreadSafetyMode? _mode;
public int? Id {
get { return _id; }
}
public T Value {
get {
if (_Lazy == null)
return _value;
else
return _Lazy.Value;
}
}
public bool IsValueCreated {
get {
if (_Lazy == null)
return true;
else
return _Lazy.IsValueCreated;
}
}
private T doCreate() {
return _createEntity(Id.Value);
}
public LazyProperty(CreateProperty creator, GetId getId)
: this(creator, getId, null, null) { }
public LazyProperty(CreateProperty creator, GetId getId, bool isThreadSafe)
: this(creator, getId, isThreadSafe, null) { }
public LazyProperty(CreateProperty creator, GetId getId, LazyThreadSafetyMode mode)
: this(creator, getId, null, mode) { }
private LazyProperty(CreateProperty creator, GetId getId, bool? isThreadSafe, LazyThreadSafetyMode? mode) {
_createEntity = creator;
_isThreadSafe = isThreadSafe;
_mode = mode;
_GetId = getId;
doSet(null, null);
}
public void Set(int? id) {
doSet(id, null);
}
public void Set(T value) {
int? id = null;
if (value != null)
id = _GetId(value);
doSet(id, id == null ? null : value);
}
private void doSet(int? id, T value) {
_id = id;
_value = value;
if (id == null || value != null)
_Lazy = null;
else if (_mode.HasValue)
_Lazy = new Lazy(doCreate, _mode.Value);
else if (_isThreadSafe.HasValue)
_Lazy = new Lazy(doCreate, _isThreadSafe.Value);
else
_Lazy = new Lazy(doCreate);
}
}