C# 如何从抽象泛型 class 继承 class 并使用泛型 class 作为类型参数

Question

I have the following abstract classes:

public abstract class Entry;
public abstract class Table<TEntry> where TEntry : Entry;
public abstract class Parser<TTable> where TTable : Table<Entry>;

Entry is a base class for a table entry containing a Dictionary of data.

Table is a base class for a table containing IEnumerables of Entries plus some other info.

Parser is a base class for text parsers which can read in strings and turn them into Entries for their ICollection of Entries, and can convert their ICollection to a Table.

public class MyEntry : Entry;
public class MyTable : Table<MyEntry>;
public class MyParser : Parser<MyTable>;

Upon trying to implement children of these base classes like so, I get the error CS0311: The type 'MyTable' cannot be used as type parameter 'TTable' in the generic type or method 'Parser<TTable>'. There is no implicit reference conversion from 'MyTable' to 'Table<Entry>' The type 'MyTable' cannot be used as type parameter 'TTable' in the generic type or method 'Parser<TTable>'. There is no implicit reference conversion from 'MyTable' to 'Table<Entry>' .

However, out can only be specified on generic types of interfaces. I need to use an abstract class here since there is some base class behavior that I want to be protected, not public.

How can I fix this?

Answer 1

It looks like it is not eligible for class as msdn says :

• Variant type parameters are restricted to generic interface and generic delegate types

So variant type parameters for class are not covariant. "Covariant" means that assignment of derived class copmatible with base type:

public class Fruit
{ 
}

public class Apple : Fruit
{ }

And this code is perfectly eligible:

IEnumerable<Fruit> fruits = new List<Apple>();