从方法返回继承的类型 [英] Return an inherited type from a method

问题描述

假设我定义了以下类:

Public Class BaseClass
    ...
End Class

Public Class DerivedClass
    Inherits BaseClass

   ... Extra Fields, methods, etc ...
End Class

然后，在我的代码中，我有一个函数，其签名为:

And then, in my code, I have a function with a signature of:

Public Function DoSomething(...) As List(Of BaseClass)

当我尝试从中返回类型为List(Of DerivedClass)的对象时，出现错误:

And when I try and return an object of type List(Of DerivedClass) from it, I get the error:

Value of type 'System.Collections.Generic.List(Of BaseClass)' cannot be converted to 'System.Collections.Generic.List(Of DerivedClass)'

我不知道DerivedClass的所有多余字段都将被填充，但是它将满足我的需要.

I know not all the extra fields of the DerivedClass will be filled, but it would give me what I needed.

有没有办法做到这一点，或者这仅仅是不良的编程习惯?而且，如果是这样，正确的方法是什么?

Is there a way to do this, or is this just considered bad programming practice? And, if so, what would be the right way to do this?

推荐答案

看看这个:

http://msdn.microsoft.com/en-us/library/dd799517(v=vs.110).aspx

了解协方差和协方差会清除一些问题:)

Understanding Covariance and Contravariance will clear things up a bit :)

• 协方差

使您可以使用比最初指定的类型更具体的类型. 您可以将IEnumerable的实例(在Visual Basic中为IEnumerable(Of Derived))分配给IEnumerable类型的变量.

Enables you to use a more specific type than originally specified. You can assign an instance of IEnumerable (IEnumerable(Of Derived) in Visual Basic) to a variable of type IEnumerable.

示例:

IEnumerable<Derived> d = new List<Derived>();
IEnumerable<Base> b = d;

• 矛盾性
• Contravariance

使您可以使用比最初指定的类型更为通用(派生较少)的类型. 您可以将IEnumerable的实例(在Visual Basic中为IEnumerable(Of Base))分配给IEnumerable类型的变量.

Enables you to use a more generic (less derived) type than originally specified. You can assign an instance of IEnumerable (IEnumerable(Of Base) in Visual Basic) to a variable of type IEnumerable.

示例:

Action<Base> b = (target) => { Console.WriteLine(target.GetType().Name); };
Action<Derived> d = b;
d(new Derived());

• 不变性
• Invariance

意味着您只能使用最初指定的类型；因此，不变的泛型类型参数既不是协变也不是协变.您不能将IEnumerable的实例(在Visual Basic中为IEnumerable(Of Base))分配给IEnumerable类型的变量，反之亦然.

Means that you can use only the type originally specified; so an invariant generic type parameter is neither covariant nor contravariant. You cannot assign an instance of IEnumerable (IEnumerable(Of Base) in Visual Basic) to a variable of type IEnumerable or vice versa.

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