除了使用访问者模式之外，有没有办法在C＃中使用变体？

Question

在C＃中没有对变体类型 （也称为标记的联合，区分联合）的直接支持。 然而，人们可以使用访问者模式 ，通过双重调度实现歧视，并保证在编译时解决所有情况。 然而，实施起来很繁琐。 我想知道是否有更容易获得的方法： 某种具有歧视机制的变体可以保证在C＃的编译时解决所有联合的情况？

// This is a variant type. At each single time it can only hold one case (a value)
// from a predefined set of cases. All classes that implement this interface
// consitute the set of the valid cases of the variant. So at each time a variant can
// be an instance of one of the classes that implement this interface. In order to
// add a new case to the variant there must be another class that implements
// this interface.
public interface ISomeAnimal
{
    // This method introduces the currently held case to whoever uses/processes
    // the variant. By processing we mean that the case is turned into a resulting
    // value represented by the generic type TResult.
    TResult GetProcessed<TResult>(ISomeAnimalProcessor<TResult> processor);
}

// This is the awkward part, the visitor that is required every time we want to
// to process the variant. For each possible case this processor has a corresponding
// method that turns that case to a resulting value.
public interface ISomeAnimalProcessor<TResult>
{
    TResult ProcessCat(Cat cat);
    TResult ProcessFish(Fish fish);
}

// A case that represents a cat from the ISomeAnimal variant.
public class Cat : ISomeAnimal
{
    public CatsHead Head { get; set; }
    public CatsBody Body { get; set; }
    public CatsTail Tail { get; set; }
    public IEnumerable<CatsLeg> Legs { get; set; }
    public TResult GetProcessed<TResult>(ISomeAnimalProcessor<TResult> processor)
    {
        // a processor has a method for each case of a variant, for this
        // particular case (being a cat) we always pick the ProcessCat method
        return processor.ProcessCat(this);
    }
}

// A case that represents a fish from the ISomeAnimal variant.
public class Fish : ISomeAnimal
{
    public FishHead Head { get; set; }
    public FishBody Body { get; set; }
    public FishTail Tail { get; set; }
    public TResult GetProcessed<TResult>(ISomeAnimalProcessor<TResult> processor)
    {
        // a processor has a method for each case of a variant, for this
        // particular case (being a fish) we always pick the ProcessCat method
        return processor.ProcessFish(this);
    }
}

public static class AnimalPainter
{
    // Now, in order to process a variant, in this case we want to
    // paint a picture of whatever animal it prepresents, we have to
    // create a new implementation of ISomeAnimalProcessor interface
    // and put the painting logic in it. 
    public static void AddAnimalToPicture(Picture picture, ISomeAnimal animal)
    {
        var animalToPictureAdder = new AnimalToPictureAdder(picture);
        animal.GetProcessed(animalToPictureAdder);
    }

    // Making a new visitor every time you need to process a variant:
    // 1. Requires a lot of typing.
    // 2. Bloats the type system.
    // 3. Makes the code harder to maintain.
    // 4. Makes the code less readable.
    private class AnimalToPictureAdder : ISomeAnimalProcessor<Nothing>
    {
        private Picture picture;

        public AnimalToPictureAdder(Picture picture)
        {
            this.picture = picture;
        }

        public Nothing ProcessCat(Cat cat)
        {
            this.picture.AddBackground(new SomeHouse());
            this.picture.Add(cat.Body);
            this.picture.Add(cat.Head);
            this.picture.Add(cat.Tail);
            this.picture.AddAll(cat.Legs);
            return Nothing.AtAll;
        }

        public Nothing ProcessFish(Fish fish)
        {
            this.picture.AddBackground(new SomeUnderwater());
            this.picture.Add(fish.Body);
            this.picture.Add(fish.Tail);
            this.picture.Add(fish.Head);
            return Nothing.AtAll;
        }
    }

}

Answer 1

你在寻找Boost Variants的东西吗？ 如果是这样，我认为不可能直接移植，因为C ++模板语言和C＃泛型有些不同。 此外， boost::variant使用访问者模式。 无论如何，如果你愿意，你可以写类似的东西。 例如（请注意，此代码仅是概念证明），您可以为访问者和变体定义两种通用类型：

public interface VariantVisitor<T, U>
{
    void Visit(T item);
    void Visit(U item);
}

public class Variant<T, U>
{
    public T Item1 { get; private set; }
    private bool _item1Set;
    public U Item2 { get; private set; }
    private bool _item2Set;

    public Variant()
    {
    }

    public void Set(T item)
    {
        this.Item1 = item;
        _item1Set = true;
        _item2Set = false;
    }

    public void Set(U item)
    {
        this.Item2 = item;
        _item1Set = false;
        _item2Set = true;
    }

    public void ApplyVisitor(VariantVisitor<T, U> visitor)
    {
        if (_item1Set)
        {
            visitor.Visit(this.Item1);
        }
        else if (_item2Set)
        {
            visitor.Visit(this.Item2);
        }
        else
        {
            throw new InvalidOperationException("Variant not set");
        }
    }
}

你可以使用这样的类型：

private static object _result;

internal class TimesTwoVisitor : VariantVisitor<int, string>
{
    public void Visit(int item)
    {
        _result = item * 2;
    }

    public void Visit(string item)
    {
        _result = item + item;
    }
}

[Test]
public void TestVisitVariant()
{
    var visitor = new TimesTwoVisitor();
    var v = new Variant<int, string>();

    v.Set(10);
    v.ApplyVisitor(visitor);
    Assert.AreEqual(20, _result);

    v.Set("test");
    v.ApplyVisitor(visitor);
    Assert.AreEqual("testtest", _result);

    var v2 = new Variant<double, DateTime>();
    v2.Set(10.5);
    //v2.ApplyVisitor(visitor);
    // Argument 1: cannot convert from 'TestCS.TestVariant.TimesTwoVisitor' to 'TestCS.TestVariant.VariantVisitor<double,System.DateTime>'
}

这样，编译器可以验证您是否将正确的访问者传递给正确的变体，并且VariantVisitor接口强制您为变体的所有类型实现Visit方法。 显然，您还可以使用两个以上的参数定义变体：

public interface VariantVisitor<T, U, V>
...
public interface VariantVisitor<T, U, V, W>
...

public class Variant<T, U, V>
...
public class Variant<T, U, V, W>
...

但我个人不喜欢这种方法，我宁愿将Visit方法转换为lambdas，并在需要时将它们作为参数传递，如上面的评论中所指出的那样。 例如，您可以编写某种穷人的模式匹配，将此方法添加到Variant<T, U> ：

    public R Match<R>(Func<T, R> f1, Func<U, R> f2)
    {
        if (_item1Set)
        {
            return f1(this.Item1);
        }
        else if (_item2Set)
        {
            return f2(this.Item2);
        }
        else
        {
            throw new InvalidOperationException("Variant not set");
        }
    }

并像这样使用它：

[Test]
public void TestMatch()
{
    var v = new Variant<int, string>();

    v.Set(10);
    var r1 = v.Match(
        i => i * 2,
        s => s.Length);
    Assert.AreEqual(20, r1);

    v.Set("test");
    var r2 = v.Match(
        i => i.ToString(),
        s => s + s);
    Assert.AreEqual("testtest", r2);
}

但请注意， 真正的模式匹配具有更多功能：警卫，详尽检查，脆弱的模式匹配检查等。

Answer 2

没门。 没有像在编译时使用访问者模式这样的概念，因为访问者模式的实现是在运行时通过在运行时使用多态，双调度和对象实例来实例化类来运行的。 Double-dispatching只能在运行时在实际对象实例上运行，它与编译时间无关。 此外，“识别机制”必须在您的对象上运行，如果您正在谈论对象，那么您在运行时...

Answer 3

我发现了一些可能对你有帮助的文章：

在C＃中： http ： //siliconcoding.wordpress.com/2012/10/26/either_in_csharp/

受歧视的工会（I）： http ： //www.drdobbs.com/cpp/discriminated-unions-i/184403821

受歧视的工会（II）： http ： //www.drdobbs.com/cpp/discriminated-unions-ii/184403828

Answer 4

所以我最终使用了一堆代表而不是访问者界面。 这是一种方法的变体，这里的一些人早先已经提出过。 显然它为我节省了一个课，一个用手关闭的麻烦，最终我必须比访客前打的少得多。 只要正确实现了GetProcessed方法，就可以保证详尽无遗（所有正在考虑的情况）。 唯一的麻烦是C＃具有“void”（缺少结果值）的东西，它由标称类型Nothing代表没有值。

// This is a variant type. At each single time it can hold one case (a value)
// from a predefined set of cases. All classes that implement this interface
// consitute the set of the valid cases of the variant. So in order to
// add a new case to the variant there must be another class that implements
// this interface.
public interface ISomeAnimal
{
    // This method introduces any possible case the variant can hold to a processing
    // function that turns the value of that case into some result.
    // Using delegates instead of an interface saves us a lot of typing!
    TResult GetProcessed<TResult>(
        Func<Cat, TResult> processCat,
        Func<Fish, TResult> processFish
    );
}

// A case that represents a cat from the ISomeAnimal variant.
public class Cat : ISomeAnimal
{
    public CatsHead Head { get; set; }
    public CatsBody Body { get; set; }
    public CatsTail Tail { get; set; }
    public IEnumerable<CatsLeg> Legs { get; set; }
    public TResult GetProcessed<TResult>(
        Func<Cat, TResult> processCat,
        Func<Fish, TResult> processFish
    ) {
        // for this particular case (being a cat) we pick the processCat delegate
        return processCat(this);
    }
}

// A case that represents a fish from the ISomeAnimal variant.
public class Fish : ISomeAnimal
{
    public FishHead Head { get; set; }
    public FishBody Body { get; set; }
    public FishTail Tail { get; set; }
    public TResult GetProcessed<TResult>(
        Func<Cat, TResult> processCat,
        Func<Fish, TResult> processFish
    ) {
        // for this particular case (being a fish) we pick the processFish method
        return processFish(this);
    }
}

public static class AnimalPainter
{
    // Now, in order to process a variant, in this case we stil want to
    // add an animal to a picture, we don't need a visitor anymore.
    // All the painting logic stays within the same method.
    // Which is:
    // 1. Much less typing.
    // 2. More readable.
    // 3. Easier to maintain.
    public static void AddAnimalToPicture(Picture picture, ISomeAnimal animal)
    {
        animal.GetProcessed<Nothing>(
            cat =>
            {
                picture.AddBackground(new SomeHouse());
                picture.Add(cat.Body);
                picture.Add(cat.Head);
                picture.Add(cat.Tail);
                picture.AddAll(cat.Legs);
                return Nothing.AtAll;
            },
            fish =>
            {
                picture.AddBackground(new SomeUnderwater());
                picture.Add(fish.Body);
                picture.Add(fish.Tail);
                picture.Add(fish.Head);
                return Nothing.AtAll;
            }
        );
    }

Answer 5

表示要对对象结构的元素执行的操作。 访问者允许您定义新操作，而无需更改其操作的元素的类。

此结构代码演示了访问者模式，其中对象遍历对象结构并在此结构中的每个节点上执行相同的操作。 不同的访客对象定义不同的操作

使用系统; 使用System.Collections;

class MainApp {static void Main（）{//设置结构ObjectStructure o = new ObjectStructure（）; o.Attach（new ConcreteElementA（））; o.Attach（new ConcreteElementB（））;

  // Create visitor objects 
  ConcreteVisitor1 v1 = new ConcreteVisitor1();
  ConcreteVisitor2 v2 = new ConcreteVisitor2();

  // Structure accepting visitors 
  o.Accept(v1);
  o.Accept(v2);

  // Wait for user 
  Console.Read();
}

}

//“访客”抽象类访客{public abstract void VisitConcreteElementA（ConcreteElementA concreteElementA）; public abstract void VisitConcreteElementB（ConcreteElementB concreteElementB）; }

//“ConcreteVisitor1”类ConcreteVisitor1：访客{public override void VisitConcreteElementA（ConcreteElementA concreteElementA）{Console.WriteLine（“{0}访问过的{0}”，concreteElementA.GetType（）。Name，this.GetType（）。Name） ; }

public override void VisitConcreteElementB(
  ConcreteElementB concreteElementB)
{
  Console.WriteLine("{0} visited by {1}",
    concreteElementB.GetType().Name, this.GetType().Name);
}

}

//“ConcreteVisitor2”类ConcreteVisitor2：访客{public override void VisitConcreteElementA（ConcreteElementA concreteElementA）{Console.WriteLine（“{0}访问过的{0}”，concreteElementA.GetType（）。Name，this.GetType（）。Name） ; }

public override void VisitConcreteElementB(
  ConcreteElementB concreteElementB)
{
  Console.WriteLine("{0} visited by {1}",
    concreteElementB.GetType().Name, this.GetType().Name);
}

}

//“元素”抽象类元素{public abstract void Accept（访问者访问者）; }

//“ConcreteElementA”类ConcreteElementA：Element {public override void Accept（Visitor visitor）{visitor.VisitConcreteElementA（this）; }

public void OperationA()
{
}

}

//“ConcreteElementB”类ConcreteElementB：Element {public override void Accept（Visitor visitor）{visitor.VisitConcreteElementB（this）; }

public void OperationB()
{
}

}

//“ObjectStructure”类ObjectStructure {private ArrayList elements = new ArrayList（）;

public void Attach(Element element)
{
  elements.Add(element);
}

public void Detach(Element element)
{
  elements.Remove(element);
}

public void Accept(Visitor visitor)
{
  foreach (Element e in elements)
  {
    e.Accept(visitor);
  }
}

}