Handling classes inherent from abstract class and type parameter

Question

I have a base abstract class and its abstract type parameter as:

public abstract class Database<T> where T : DatabaseItem, new() {
  protected List<T> _items = new List<T> ();
  protected virtual void Read (string[] cols)   {
    T item = new T ();
    ...
}

public abstract class DatabaseItem {
  ...
}

Then I have number of children classes inherent from it:

public class ShopDatabase : Database<ShopItem> {}
public class ShopItem : DatabaseItem {}

public class WeaponDatabase : Database<WeaponItem> {}
public class WeaponItem : DatabaseItem {}

...

Now, I want to put a Dictionary to keep track of the databases and a method to return them using DatabaseItem type, something like this:

Dictionary<Type, Database<DatabaseItem>> databases;

public static Database<T> GetDatabase<T> () where T: DatabaseItem {
  return databasebases [typeof (T)];
}

Then it gave me "'T' must be a non-abstract type with a public parameterless constructor in order to use it as parameter 'T'" error because DatabaseItem is abstract. I made DatabaseItem as non-abstract type, the error is gone but still lot of type conversion errors came out...

Found a similar question but I still not understand...

What's the better solution/structure to achieve this?

Answer 1

The simplest you can do is to store Database<T> descendants as objects:

static class DatabaseManager
{
    private static Dictionary<Type, object> databases = new Dictionary<Type, object>();

    // ....

    public static Database<T> GetDatabase<T>()
        where T : DatabaseItem, new()
    {
        return (Database<T>)databases[typeof(T)];
    }
}

Even if you turn DatabaseItem to non-abstract class with parameterless constructor, this won't help either, because typeof(Database<ShopItem>) != typeof(Database<DatabaseItem>).

Note, that generic constraints for GetDatabase<T> method must be the same as for Database<T> class.

Upd.

is there a way to know what type parameter a class using? e.g. giving ShopDatabase, I want to get ShopItem. I need it when I initialize databases dictionary

Use reflection:

var databaseItemType = typeof(ShopDatabase).BaseType.GetGenericArguments()[0];

For cases like this:

class FooDatabase : Database<FooItem> {}
class BooDatabase : FooDatabase {}
// etc...

you'll need to loop through inheritance tree to find Database<FooItem>.

Answer 2

The bad news is that you can't use your code without kind of cast. Define a non-generic type or inerface for database, then use this code:

Dictionary<Type, Database> databases;
public static Database<T> GetDatabase<T>() where T: DatabaseItem, new()
{
    return databasebases[typeof(T)] as Database<T>;
}