向下和向上倾斜

我是新的 C # (和 哦)。当我有一些像下面这样的代码:

class Employee
{
// some code
}




class Manager : Employee
{
//some code
}

问题1 : 如果我有其他代码可以做到这一点:

   Manager mgr = new Manager();
Employee emp = (Employee)mgr;

这里的 Employee是一个 Manager，但是当我把它这样一个 Employee它意味着我正在向上抛出它？

问题2 :

当我有几个 Employee类对象和一些，但不是所有的是 Manager的，我怎么能向下转换他们在可能的地方？

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小开
最佳答案

  1. That is correct. When you do that you are casting it it into an employee object, so that means you cannot access anything manager specific.

  2. Downcasting is where you take a base class and then try and turn it into a more specific class. This can be accomplished with using is and an explicit cast like this: 

    if (employee is Manager)
{
Manager m = (Manager)employee;
//do something with it
}

or with the as operator like this:

Manager m = (employee as Manager);
if (m != null)
{
//do something with it
}

If anything is unclear I'll be happy to correct it!

小开

Upcasting (using (Employee)someInstance) is generally easy as the compiler can tell you at compile time if a type is derived from another.

Downcasting however has to be done at run time generally as the compiler may not always know whether the instance in question is of the type given. C# provides two operators for this - is which tells you if the downcast works, and return true/false. And as which attempts to do the cast and returns the correct type if possible, or null if not.

To test if an employee is a manager:

Employee m = new Manager();
Employee e = new Employee();


if(m is Manager) Console.WriteLine("m is a manager");
if(e is Manager) Console.WriteLine("e is a manager");

You can also use this

Employee someEmployee = e  as Manager;
if(someEmployee  != null) Console.WriteLine("someEmployee (e) is a manager");


Employee someEmployee = m  as Manager;
if(someEmployee  != null) Console.WriteLine("someEmployee (m) is a manager");
小开

In case you need to check each of the Employee object whether it is a Manager object, use the OfType method:

List<Employee> employees = new List<Employee>();


//Code to add some Employee or Manager objects..


var onlyManagers = employees.OfType<Manager>();


foreach (Manager m in onlyManagers) {
// Do Manager specific thing..
}
小开
  • Upcasting is an operation that creates a base class reference from a subclass reference. (subclass -> superclass) (i.e. Manager -> Employee)
  • Downcasting is an operation that creates a subclass reference from a base class reference. (superclass -> subclass) (i.e. Employee -> Manager)

In your case

Employee emp = (Employee)mgr; //mgr is Manager

you are doing an upcasting.

An upcast always succeeds unlike a downcast that requires an explicit cast because it can potentially fail at runtime.(InvalidCastException).

C# offers two operators to avoid this exception to be thrown:

Starting from:

Employee e = new Employee();

First: 

Manager m = e as Manager; // if downcast fails m is null; no exception thrown

Second:

if (e is Manager){...} // the predicate is false if the downcast is not possible

Warning: When you do an upcast you can only access to the superclass' methods, properties etc...

小开

Upcasting and Downcasting:

Upcasting: Casting from Derived-Class to Base Class Downcasting: Casting from Base Class to Derived Class

Let's understand the same as an example:

Consider two classes Shape as My parent class and Circle as a Derived class, defined as follows:

class Shape
{
public int Width { get; set; }
public int Height { get; set; }
}


class Circle : Shape
{
public int Radius { get; set; }
public bool FillColor { get; set; }
}

Upcasting:

Shape s = new Shape();

Circle c= s;

Both c and s are referencing to the same memory location, but both of them have different views i.e using "c" reference you can access all the properties of the base class and derived class as well but using "s" reference you can access properties of the only parent class.

A practical example of upcasting is Stream class which is baseclass of all types of stream reader of .net framework:

StreamReader reader = new StreamReader(new FileStreamReader());

here, FileStreamReader() is upcasted to streadm reder.

Downcasting:

Shape s = new Circle(); here as explained above, view of s is the only parent, in order to make it for both parent and a child we need to downcast it

var c = (Circle) s;

The practical example of Downcasting is button class of WPF.

小开

Answer 1 : Yes it called upcasting but the way you do it is not modern way. Upcasting can be performed implicitly you don't need any conversion. So just writing Employee emp = mgr; is enough for upcasting.

Answer 2 : If you create object of Manager class we can say that manager is an employee. Because class Manager : Employee depicts Is-A relationship between Employee Class and Manager Class. So we can say that every manager is an employee.

But if we create object of Employee class we can not say that this employee is manager because class Employee is a class which is not inheriting any other class. So you can not directly downcast that Employee Class object to Manager Class object.

So answer is, if you want to downcast from Employee Class object to Manager Class object, first you must have object of Manager Class first then you can upcast it and then you can downcast it.


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