为什么c#是这样设计的?

根据我的理解,一个接口只描述行为,并且服务于描述实现接口的类的契约义务。

如果类希望在共享方法中实现这种行为,为什么不应该呢?

以下是我想到的一个例子:

// These items will be displayed in a list on the screen.
public interface IListItem {
  string ScreenName();
  ...
}

public class Animal: IListItem {
    // All animals will be called "Animal".
    public static string ScreenName() {
        return "Animal";
    }
....
}

public class Person: IListItem {

    private string name;

    // All persons will be called by their individual names.
    public string ScreenName() {
        return name;
    }

    ....

 }

当前回答

因为接口的目的是允许多态性,能够传递任意数量的已定义类的实例,这些类都已定义,以实现已定义的接口……确保在多态调用中,代码能够找到您正在调用的方法。允许静态方法实现接口是没有意义的,

你怎么称呼它??


public interface MyInterface { void MyMethod(); }
public class MyClass: MyInterface
{
    public static void MyMethod() { //Do Something; }
}

 // inside of some other class ...  
 // How would you call the method on the interface ???
    MyClass.MyMethod();  // this calls the method normally 
                         // not through the interface...

    // This next fails you can't cast a classname to a different type... 
    // Only instances can be Cast to a different type...
    MyInterface myItf = MyClass as MyInterface;  

其他回答

这里的大多数答案似乎都没有抓住重点。多态性不仅可以在实例之间使用,还可以在类型之间使用。当我们使用泛型时,这是经常需要的。

假设我们在泛型方法中有类型参数,我们需要对它做一些操作。我们不想实例化,因为我们不知道构造函数。

例如:

Repository GetRepository<T>()
{
  //need to call T.IsQueryable, but can't!!!
  //need to call T.RowCount
  //need to call T.DoSomeStaticMath(int param)
}

...
var r = GetRepository<Customer>()

不幸的是,我只能想出一些“丑陋”的选择:

Use reflection Ugly and beats the idea of interfaces and polymorphism. Create completely separate factory class This might greatly increase the complexity of the code. For example, if we are trying to model domain objects, each object would need another repository class. Instantiate and then call the desired interface method This can be hard to implement even if we control the source for the classes, used as generic parameters. The reason is that, for example we might need the instances to be only in well-known, "connected to DB" state.

例子:

public class Customer 
{
  //create new customer
  public Customer(Transaction t) { ... }

  //open existing customer
  public Customer(Transaction t, int id) { ... }

  void SomeOtherMethod() 
  { 
    //do work...
  }
}

为了使用实例化来解决静态接口问题,我们需要做以下事情:

public class Customer: IDoSomeStaticMath
{
  //create new customer
  public Customer(Transaction t) { ... }

  //open existing customer
  public Customer(Transaction t, int id) { ... }

  //dummy instance
  public Customer() { IsDummy = true; }

  int DoSomeStaticMath(int a) { }

  void SomeOtherMethod() 
  { 
    if(!IsDummy) 
    {
      //do work...
    }
  }
}

这显然是丑陋的,也是不必要的,会使所有其他方法的代码复杂化。显然,这也不是一个优雅的解决方案!

您可以将类的静态方法和非静态方法看作是不同的接口。调用时,静态方法解析为单例静态类对象,而非静态方法解析为所处理的类的实例。所以,如果你在一个接口中使用静态和非静态方法,你实际上是在声明两个接口,而实际上我们想要接口被用来访问一个内聚的东西。

因为接口是继承结构,静态方法继承不好。

Regarding static methods used in non-generic contexts I agree that it doesn't make much sense to allow them in interfaces, since you wouldn't be able to call them if you had a reference to the interface anyway. However there is a fundamental hole in the language design created by using interfaces NOT in a polymorphic context, but in a generic one. In this case the interface is not an interface at all but rather a constraint. Because C# has no concept of a constraint outside of an interface it is missing substantial functionality. Case in point:

T SumElements<T>(T initVal, T[] values)
{
    foreach (var v in values)
    {
        initVal += v;
    }
}

这里没有多态性,泛型使用对象的实际类型并调用+=操作符,但这失败了,因为它不能确定该操作符是否存在。简单的解决方案是在约束中指定它;简单的解决方案是不可能的,因为操作符是静态的,静态方法不能在接口中,(这就是问题所在)约束被表示为接口。

c#需要的是一个真正的约束类型,所有的接口都是约束,但不是所有的约束都是接口,然后你可以这样做:

constraint CHasPlusEquals
{
    static CHasPlusEquals operator + (CHasPlusEquals a, CHasPlusEquals b);
}

T SumElements<T>(T initVal, T[] values) where T : CHasPlusEquals
{
    foreach (var v in values)
    {
        initVal += v;
    }
}

已经有很多关于为所有数字类型创建i算术来实现的讨论,但是存在效率问题,因为约束不是多态构造,所以创建算术约束可以解决这个问题。

接口是一个OOPS概念,这意味着接口的每个成员都应该通过对象或实例得到使用。因此,接口不能有静态方法。