使用通用Collection类获取其他类的Collection

Question

I have a collection class, let's say UserCollection , which is derived from Collection (base class for a generic collection).

Now I want a UserCollection and here is my code, but it is saying it can not convert from generic List to my collection class:

public class User
{
  // All user Fields
}

public class UserCollection : Collection<User>
{
   public static UserCollection GetUserCollection()
   {
       DataTable table = ...//all users data from DB
       List<User> userList = table.ToCollection<User>(); // Call an extension method to convert to          generic List of user
       return userList; // Cannot implicitly convert type 'System.Collections.Generic.List<User>' to 'UserCollection'
   }
}

// ToCollection function is here

http://www.codeproject.com/Tips/195889/Convert-Datatable-to-Collection-using-Generic

What do I need to make it work?

Answer 1

You could provide an appropriate constructor that calls the base constructor:

public class UserCollection : Collection<User>
{
    public UserCollection() { }
    public UserCollection(IList<User> users) : base(users) { }

    public static UserCollection GetUserCollection()
    {
        // ...
        List<User> userList = ... 
        return new UserCollection(userList);
    }
}

or you have to return ICollection<User> instead because List<User> is not a child class of Collection<User> or UserCollection but it implements ICollection<User> .

Here is a related question: Cannot implicitly convert List<T> to Collection<T>

Answer 2

There are two reasons this will not work. Primarily, you cannot cast a class to an inherited class. UserCollection is a Collection, but Collection is not a UserCollection. You could assign UserCollection to a Collection variable but no the other way around. Additionally, you are receiving a List from the ToCollection method. You run into the same scenario: Collection<> is not a List<>. See http://msdn.microsoft.com/en-us/library/ms173105.aspx for more information about casting.

One solution is to implement a constructor that calls the base constructor that takes an IList<> like so:

public UserCollection(IList<User> users)
    : base(users)
{
}

Then you could modify your GetUserCollection method to call it:

public static UserCollection GetUserCollection()
{
   DataTable table = ...//all users data from DB
   List<User> userList = table.ToCollection<User>(); 
   return new UserCollection(userList);
}