Generic function throws cast exception

Question

I have a weird problem. :(

I get an enumerable object from my database and want to convert the objects either to a list of base class objects or to a list of fully blown derived class objects . In some situations I want to cut of attributes I do not need - so standard polymorphism is not the way for me. This function resides in an web service and I have to optimize traffic.

Therefore I wrote a generic function for casting the whole list to the type I want. But in the case of the derived type I get an cast error. But I don't understand the message because the source object has not the type, the message is telling me.

Any ideas what is going wrong? Thx in advance!

using System;
using System.Collections.Generic;
using System.Linq;

class DBClass {
    public string id { get; set; }
    public string name { get; set; }

    public DBClass() { id = Guid.NewGuid().ToString(); }
}

class BaseClass {
    public Guid ID { get; set; }

    public static explicit operator BaseClass(DBClass x) {
        return new BaseClass() {
            ID = new Guid(x.id)
        };
    }
}

class DerivedClass : BaseClass {
    public string Name { get; set; }

    public static explicit operator DerivedClass(DBClass x) {
        return new DerivedClass() {
            ID = new Guid(x.id),
            Name = x.name
        };
    }
}

class Program
{
    static List<BaseClass> convertToList<T>(IEnumerable<DBClass> xx) where T : BaseClass
    {
        // solution from Jeroen Mostert (see comments):
        return xx.Select(obj => (BaseClass) (T) (dynamic) obj).ToList();

        // original code with error
        var l = xx.Select( obj => (T) obj);
        // T == BaseClass: everything is fine
        // T == DerivedClass: Unable to cast object of type 'BaseClass' to type 'DerivedClass'.
        // but obj is of type DBClass!

        return l.ToList() as List<BaseClass>;
    }

    static void Main(string[] args)
    {
        List < DBClass >  dbValues = new List<DBClass>();
        dbValues.Add(new DBClass());
        dbValues.Add(new DBClass());
        dbValues.Add(new DBClass());

        convertToList<BaseClass>(dbValues);     // ok
        convertToList<DerivedClass>(dbValues);  // KO
    }
}