我试图了解c#中的嵌套类。我理解嵌套类是在另一个类中定义的类,我不明白的是为什么我需要这样做。
其目的通常只是限制嵌套类的范围。与普通类相比,嵌套类具有私有修饰符的额外可能性(当然也是受保护的)。
Basically, if you only need to use this class from within the "parent" class (in terms of scope), then it is usually appropiate to define it as a nested class. If this class might need to be used from without the assembly/library, then it is usually more convenient to the user to define it as a separate (sibling) class, whether or not there is any conceptual relationship between the two classes. Even though it is technically possible to create a public class nested within a public parent class, this is in my opinion rarely an appropiate thing to implement.
嵌套类可以具有私有、受保护和受保护的内部访问修饰符以及公共和内部访问修饰符。
例如,您正在实现GetEnumerator()方法,该方法返回一个IEnumerator<T>对象。消费者不会关心对象的实际类型。他们只知道它实现了那个接口。要返回的类没有任何直接用途。你可以声明这个类为私有嵌套类,并返回它的一个实例(这实际上是c#编译器实现迭代器的方式):
class MyUselessList : IEnumerable<int> {
// ...
private List<int> internalList;
private class UselessListEnumerator : IEnumerator<int> {
private MyUselessList obj;
public UselessListEnumerator(MyUselessList o) {
obj = o;
}
private int currentIndex = -1;
public int Current {
get { return obj.internalList[currentIndex]; }
}
public bool MoveNext() {
return ++currentIndex < obj.internalList.Count;
}
}
public IEnumerator<int> GetEnumerator() {
return new UselessListEnumerator(this);
}
}
有时候,实现一个将从类内部返回的接口是有用的,但该接口的实现应该完全对外界隐藏。
举个例子,在将yield添加到c#之前,实现枚举器的一种方法是将枚举器的实现作为一个集合中的私有类。这将为访问集合的成员提供方便,但外部世界不需要/看到这是如何实现的细节。
嵌套类对于实现不应该公开的内部细节非常有用。如果你使用Reflector检查Dictionary<Tkey,TValue>或Hashtable之类的类,你会发现一些例子。
我不明白的是我为什么要这么做
我觉得你根本不需要这么做。给定一个这样的嵌套类…
class A
{
//B is used to help implement A
class B
{
...etc...
}
...etc...
}
... 你总是可以移动内部/嵌套类到全局作用域,像这样…
class A
{
...etc...
}
//B is used to help implement A
class B
{
...etc...
}
然而,当B仅用于帮助实现A时,则使B成为内部/嵌套类有两个优点:
It doesn't pollute the global scope (e.g. client code which can see A doesn't know that the B class even exists) The methods of B implicitly have access to private members of A; whereas if B weren't nested inside A, B wouldn't be able to access members of A unless those members were internal or public; but then making those members internal or public would expose them to other classes too (not just B); so instead, keep those methods of A private and let B access them by declaring B as a nested class. If you know C++, this is like saying that in C# all nested classes are automatically a 'friend' of the class in which they're contained (and, that declaring a class as nested is the only way to declare friendship in C#, since C# doesn't have a friend keyword).
当我说B可以访问A的私有成员时,这是假设B有一个对A的引用;它经常这样做,因为嵌套类通常是这样声明的…
class A
{
//used to help implement A
class B
{
A m_a;
internal B(A a) { m_a = a; }
...methods of B can access private members of the m_a instance...
}
...etc...
}
... 并使用这样的代码从a的方法构建…
//create an instance of B, whose implementation can access members of self
B b = new B(this);
你可以在Mehrdad的回复中看到一个例子。
公共嵌套成员也有很好的用途…
嵌套类可以访问外部类的私有成员。因此,在一个场景中,这是正确的方式将是创建一个比较器(即。实现IComparer接口)。
在这个例子中,FirstNameComparer可以访问私有的_firstName成员,如果类是一个单独的类,它就不能访问。
public class Person
{
private string _firstName;
private string _lastName;
private DateTime _birthday;
//...
public class FirstNameComparer : IComparer<Person>
{
public int Compare(Person x, Person y)
{
return x._firstName.CompareTo(y._firstName);
}
}
}
我特别喜欢的一个模式是将嵌套类与工厂模式结合起来:
public abstract class BankAccount
{
private BankAccount() {} // prevent third-party subclassing.
private sealed class SavingsAccount : BankAccount { ... }
private sealed class ChequingAccount : BankAccount { ... }
public static BankAccount MakeSavingAccount() { ... }
public static BankAccount MakeChequingAccount() { ... }
}
通过像这样嵌套类,我使第三方不可能创建他们自己的子类。我可以完全控制在任何bankaccount对象中运行的所有代码。我的所有子类都可以通过基类共享实现细节。
也许这是一个何时使用嵌套类的好例子?
// ORIGINAL
class ImageCacheSettings { }
class ImageCacheEntry { }
class ImageCache
{
ImageCacheSettings mSettings;
List<ImageCacheEntry> mEntries;
}
And:
// REFACTORED
class ImageCache
{
Settings mSettings;
List<Entry> mEntries;
class Settings {}
class Entry {}
}
PS:我没有考虑应该应用哪些访问修饰符(私有,受保护,公共,内部)
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