C＃中没有variant types（也称为标记的联合，歧视联盟）的直接支持。然而，可以使用visitor pattern来实现通过双重调度进行区分，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＃编译时解决所有联合案例？
// 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;
}
}
}
答案 0 :(得分：4)
您是否正在寻找Boost Variants的内容？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了正确的变体，VariantVisitor界面强制您为变体的所有类型实现Visit方法。显然，您还可以定义具有两个以上参数的变体：
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>
...
但就我个人而言，我不喜欢这种方法，我宁愿将Visit方法转换为lambda，并将其作为参数传递给所需的参数，如上面的评论所述。例如，您可以编写某种穷人的模式匹配，将此方法添加到类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。
答案 1 :(得分：2)
没办法。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，“辨别机制”必须在你的对象上运行，如果你在谈论对象，你就是在运行时..
答案 2 :(得分：1)
我发现了一些可能对你有帮助的文章：
在C＃：http://siliconcoding.wordpress.com/2012/10/26/either_in_csharp/
中受歧视的工会（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
答案 3 :(得分：0)
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，表示没有值。
// 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;
}
);
}
答案 4 :(得分：-1)
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了不同的操作。
使用System; 使用System.Collections;
类MainApp { static void Main（） { //安装结构 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” class 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” class 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接受（访客访客）; }
//“ConcreteElementA” class ConcreteElementA：Element { public override void Accept（访客访客） { visitor.VisitConcreteElementA（本）; }
public void OperationA()
{
}
}
//“ConcreteElementB” class ConcreteElementB：Element { public override void Accept（访客访客） { visitor.VisitConcreteElementB（本）; }
public void OperationB()
{
}
}
//“ObjectStructure” class 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);
}
}
}