Question

我试图建立一个流畅的界面，其中包含许多扩展基本描述符的泛型和描述符。我把它放在一个github仓库中，因为在这里粘贴所有代码会让它变得难以理解。

在阅读了Eric Lippert关于类型约束的帖子（http://blogs.msdn.com/b/ericlippert/archive/2009/12/10/constraints-are-not-part-of-the-signature.aspx）和阅读No type inference with generic extension method之后，我对这个主题的理解有所改善，但我仍然有疑问。

假设您有一些允许流畅调用的类：

var giraffe = new Giraffe();
new ZooKeeper<Giraffe>()
    .Name("Jaap")
    .FeedAnimal(giraffe);

var reptile = new Reptile();
new ExperiencedZooKeeper<Reptile>()
    .Name("Martijn")
    .FeedAnimal(reptile)
    .CureAnimal(reptile);

这些课程如下：

public class ZooKeeper<T>
    where T : Animal
{
    internal string name;
    internal List<T> animalsFed = new List<T>();

    // this method needs to be fluent
    public ZooKeeper<T> Name(string name)
    {
        this.name = name;
        return this;
    }

    // this method needs to be fluent
    public ZooKeeper<T> FeedAnimal(T animal)
    {
        animalsFed.Add(animal);
        return this;
    }
}

public class ExperiencedZooKeeper<T> : ZooKeeper<T>
    where T : Animal
{
    internal List<T> animalsCured = new List<T>();

    // this method needs to be fluent
    // but we must new it in order to be able to call CureAnimal after this
    public new ExperiencedZooKeeper<T> Name(string name)
    {
        base.Name(name);
        return this;
    }

    // this method needs to be fluent
    // but we must new it in order to be able to call CureAnimal after this
    public new ExperiencedZooKeeper<T> FeedAnimal(T animal)
    {
        base.FeedAnimal(animal);
        return this;
    }

    // this method needs to be fluent
    public ExperiencedZooKeeper<T> CureAnimal(T animal)
    {
        animalsCured.Add(animal);
        return this;
    }
}

我试图摆脱新的＆＃39;隐藏ExperiencedZooKeeper实现的ZooKeeper中的方法。区别在于new中的ExperiencedZooKeeper方法返回正确的类型。没有new方法，AFAIK就无法做到这一点。

我尝试采取的另一种方法是移动“安装人员”。扩展方法。这适用于.Name（）方法，但它引入了一个包含内部字段的ZooKeeperBase：

public abstract class ZooKeeperBase
{
    internal string name;

}

public class ZooKeeper<T> : ZooKeeperBase
    where T : Animal
{
    internal List<T> animalsFed = new List<T>();


    // this method needs to be fluent
    public ZooKeeper<T> FeedAnimal(T animal)
    {
        animalsFed.Add(animal);
        return this;
    }
}

public static class ZooKeeperExtensions
{

    // this method needs to be fluent
    public static TZooKeeper Name<TZooKeeper>(this TZooKeeper zooKeeper, string name)
        where TZooKeeper : ZooKeeperBase
    {
        zooKeeper.name = name;
        return zooKeeper;
    }
}

但是这种精确的方法对于FeedAnimal（T动物）并不起作用，它需要一个额外的类型参数：

// this method needs to be fluent
public static TZooKeeper FeedAnimal<TZooKeeper, T>(this TZooKeeper zooKeeper, T animal)
    where TZooKeeper : ZooKeeper<T>
    where T : Animal
{
    zooKeeper.animalsFed.Add(animal);
    return zooKeeper;
}

这仍然可以并且效果很好，你仍然可以流利地称它：

new ExperiencedZooKeeper<Reptile>()
    .Name("Martijn")
    .FeedAnimal(reptile)
    .CureAnimal(reptile);

当我尝试使用以下方法时，真正的问题就开始了：

public static TZooKeeper Favorite<TZooKeeper, T>(this TZooKeeper zooKeeper, Func<T, bool> animalSelector)
    where TZooKeeper : ZooKeeper<T>
    where T : Animal
{
    zooKeeper.favoriteAnimal = zooKeeper.animalsFed.FirstOrDefault(animalSelector);
    return zooKeeper;
}

您无法像这样致电Favorite：

new ExperiencedZooKeeper<Reptile>()
  .Name("Eric")
  .FeedAnimal(reptile)
  .FeedAnimal(new Reptile())
  .Favorite(r => r == reptile)

因为它会导致与No type inference with generic extension method相同的问题，但是，这种情况稍微复杂一些，因为我们已经有了一个Type参数TZookKeeper，它描述了我们需要的T.但就像Eric Lipperts博客文章一样，类型约束不是签名的一部分：

The type arguments for method 'TestTypeInference5.ZooKeeperExtensions.Favorite<TZooKeeper,T>(TZooKeeper, System.Func<T,bool>)' cannot be inferred from the usage. Try specifying the type arguments explicitly.

有关完整代码，请参阅https://github.com/q42jaap/TestTypeInference 这个回购中的自述文件实际上解释了我试图解决的现实问题。

所以真正的问题是，有没有办法创建这种流畅的方法风格，而不是将ZooKeeper的每个方法都添加到ZooKeeper的每个子类中，new隐藏ZooKeeper本身的方法？

Answer 1

一种可能性是为每个级别创建一个基类，并从中派生一个空的处理程序类：

基础课程：

public abstract class ZooKeeperBase<TZooKeeper, TAnimal>
    where TZooKeeper : ZooKeeperBase<TZooKeeper, TAnimal>
    where TAnimal : Animal
{
    private string name;
    private List<TAnimal> animalsFed = new List<TAnimal>();
    private TAnimal favoriteAnimal;

    public TZooKeeper Name(string name)
    {
        this.name = name;
        return (TZooKeeper)this;
    }

    public TZooKeeper FeedAnimal(TAnimal animal)
    {
        animalsFed.Add(animal);
        return (TZooKeeper)this;
    }

    public TZooKeeper Favorite(Func<TAnimal, bool> animalSelector)
    {
        favoriteAnimal = animalsFed.FirstOrDefault(animalSelector);
        return (TZooKeeper)this;
    }
}

public abstract class ExperiencedZooKeeperBase<TZooKeeper, TAnimal>
    : ZooKeeperBase<TZooKeeper, TAnimal>
    where TZooKeeper : ExperiencedZooKeeperBase<TZooKeeper, TAnimal>
    where TAnimal : Animal
{
    private List<TAnimal> animalsCured = new List<TAnimal>();

    public TZooKeeper CureAnimal(TAnimal animal)
    {
        animalsCured.Add(animal);
        return (TZooKeeper)this;
    }
}

处理程序类：

public class ZooKeeper<T> : ZooKeeperBase<ZooKeeper<T>, T>
    where T : Animal
{
}

public class ExperiencedZooKeeper<T>
    : ExperiencedZooKeeperBase<ExperiencedZooKeeper<T>, T>
    where T : Animal
{
}

用法与您在问题中的表现一样。

扩展方法和类型推断

1 个答案: