LINQ查询异类集合中具有相同基类的不同类型

Question

是否有任何简单的方法来查询异类集合，其中集合中的对象都来自同一个基类，但有些可能是一个派生类型，有些可能是另一个？

例如，这里有一个类层次结构：

public class Ship 
{ 
    public string Name { get; set; } 
    public string Description { get; set; } 
} 

public class SailingVessel : Ship 
{ 
    public string Rig { get; set; } 
    public int NumberOfMasts { get; set; } 
} 

public class MotorVessel : Ship 
{ 
    public string Propulsion { get; set; } 
    public decimal TopSpeed { get; set; } 
} 

这里就是我想要查询XML文档：

<?xml version="1.0" ?> 
<ships xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"> 
    <ship xsi:type="sailingVessel"> 
    <name>Cutty Sark</name> 
    <description>Tea clipper</description> 
    <rig>Ship</rig> 
    <numberOfMasts>3</numberOfMasts> 
    </ship> 
    <ship xsi:type="sailingVessel"> 
    <name>Peking</name> 
    <description>Windjammer of the Flying P Line</description> 
    <rig>Barque</rig> 
    <numberOfMasts>4</numberOfMasts> 
    </ship> 
    <ship xsi:type="motorVessel"> 
    <name>HMS Hood</name> 
    <description>Last British battlecruiser</description> 
    <propulsion>SteamTurbine</propulsion> 
    <topSpeed>28</topSpeed> 
    </ship> 
    <ship xsi:type="motorVessel"> 
    <name>RMS Queen Mary 2</name> 
    <description>Last transatlantic passenger liner</description> 
    <propulsion>IntegratedElectricPropulsion</propulsion> 
    <topSpeed>30</topSpeed> 
    </ship> 
    <ship xsi:type="motorVessel"> 
    <name>USS Enterprise</name> 
    <description>First nuclear-powered aircraft carrier</description> 
    <propulsion>Nuclear</propulsion> 
    <topSpeed>33.6</topSpeed> 
    </ship> 
</ships> 

我可以查询XML文档并阅读其内容入名单发货物件：

 XDocument xmlDocument = XDocument.Load("Ships.xml") 
     XNamespace xsi = "http://www.w3.org/2001/XMLSchema-instance"; 

     var records = (from record in xmlDocument.Descendants("ship") 
         let type = record.Attribute(xsi + "type").Value       
         select new Ship 
         { 
          Name = (string)record.Element("name"), 
          Description = (string)record.Element("description") 
         }).ToArray<Ship>(); 

返回以下内容：

Ship[0] (type: Ship): 
    Name: Cutty Sark 
    Description: Tea clipper 
Ship[1] (type: Ship): 
    Name: Peking 
    Description: Windjammer of the Flying P Line 
Ship[2] (type: Ship): 
    Name: HMS Hood 
    Description: Last British battlecruiser 
Ship[3] (type: Ship): 
    Name: RMS Queen Mary 2 
    Description: Last transatlantic passenger liner 
Ship[4] (type: Ship): 
    Name: USS Enterprise 
    Description: First nuclear-powered aircraft carrier 

我真的很希望能够生产的，虽然是这样的：

Ship[0] (type: SailingVessel): 
    Name: Cutty Sark 
    Description: Tea clipper 
    Rig: Ship 
    NumberOfMasts: 3 
Ship[1] (type: SailingVessel): 
    Name: Peking 
    Description: Windjammer of the Flying P Line 
    Rig: Barque 
    NumberOfMasts: 4 
Ship[2] (type: MotorVessel): 
    Name: HMS Hood 
    Description: Last British battlecruiser 
    Propulsion: SteamTurbine 
    TopSpeed: 28 
Ship[3] (type: MotorVessel): 
    Name: RMS Queen Mary 2 
    Description: Last transatlantic passenger liner 
    Propulsion: IntegratedElectricPropulsion 
    TopSpeed: 30 
Ship[4] (type: MotorVessel): 
    Name: USS Enterprise 
    Description: First nuclear-powered aircraft carrier 
    Propulsion: Nuclear 
    TopSpeed: 33.6 

如何修改LINQ查询并初始化一个SailingVessel对象或MotorVessel对象合适，而不是基础Ship对象？

我是否必须做两个选择，并复制每个基类属性（名称和描述）的对象初始化？我能想到的就是这些，但我讨厌涉及代码的重复。或者，是否有一些方法可以初始化基类的属性，并根据需要选择初始化SailingVessel（Rig，NumberOfMasts）或MotorVessel（Propulsion，TopSpeed）的其他属性？

Answer 1

我个人给每个类型的静态FromXElement方法（或构造函数），然后创建一个Dictionary<string, Func<Ship>>这样的：

var factories = new Dictionary<string, Func<Ship>> 
{ 
    { "sailingVessel", SailingVessel.FromXElement }, 
    { "motorVessl", MotorVessel.FromXElement }, 
    ... 
}; 

然后将查询将是：

var records = from record in xmlDocument.Descendants("ship") 
       let type = record.Attribute(xsi + "type").Value 
       select factories[type](record); 

你可以给Ship类保护构造函数采取XElement来提取公共属性，留下类似的东西：

public class SailingVessel : Ship 
{ 
    public Rig Rig { get; set; } 
    public int NumberOfMasts { get; set; } 

    private SailingVessel(XElement element) : base(element) 
    { 
     Rig = (Rig) Enum.Parse(typeof(Rig), (string) element.Element("Rig")); 
     NumberOfMasts = (int) element.Element("NumberOfMasts"); 
    } 

    // Don't really need this of course - could put constructor calls 
    // into your factory instead. I like the flexibility of factory 
    // methods though, e.g. for caching etc. 
    public static FromXElement(element) 
    { 
     return new SailingVessel(element); 
    } 
} 

确实会涉及相当数量的代码，但它会相当简单，而且也容易测试。