如何在C＃中将派生类型列表分配给基类型列表？

Question

How to achieve List<Employee> e = new List<Associate>() ? Is this wrong?

class Employee
{
    public int ID { get; set; }
    public string Name { get; set; }
}

class Associate : Employee
{
}

class Compute
{
    static void Main()
    {
        List<Employee> e = new List<Associate>();
    }
}

Answer 1

Imagine you have another class that inherits from Employee as well, say,

class Manager : Employee
{
}

Then, in your example this could happen,

class Compute
{
    static void Main()
    {
        List<Employee> employees = new List<Associate>();
        employees.Add(new Manager()); // Hmmm... what?
    }
}

And suddenly you have a Manager in a list of Associate , which violate the type rules because Manager and Associate have no class relationship. That's part of the reason why List<T> has the declared the T type parameter as invariant (rather than List<out T> or covariant, where something like this could actually happen).

Answer 2

You can add derived classes to lists of base classes.

List<Employee> e= new List<Employee>();
e.Add(new Associate());

Answer 3

You can not. The reason of this behavior is that you still will be able to add base class instances to a list what should be just derived classes (as covered in blogs.msdn provided by BrunoLM).

e.Add(new Employee());

Following are possible ways to avoid the problem:

• you can have list of base class items as you can easily add instances of derived class.
• you can use var in code to implicitly define type of variable (it indeed will be of type List<Associate>
• if you need to pass this list to a function - make argument IEnumerable<Employee> (and you'll be able to pass either List<Employee> or List<Associate> )
• you can use Enumerable.OfType to filter elements of derived classes from list of elements of base class.