Returning generic lists from methods in C#

Question

I'm having a difficulty returning a generic collection from a Linq Extension. I call the method with a single parameter:


   static List<NewPeople> GetPeople()
           {
               List<People> ppl = Enttity.GetCollection();
               var abc = new List<NewPeople>();
               ppl.CopyAllTo(abc);

               return abc;
               //return nppl;
           }

Called Method:



   public static IList<U> CopyAllTo<T, U>(this IEnumerable<T> source, List<U> destination) 
                   where T : class 
                   where U : class
           {
               List<U> objectList = new List<U>();
               foreach (T t in source)
               {
                   U u = Activator.CreateInstance<U>();
                   t.CopyTo(u);
                   objectList.Add(u);
               }

               return objectList;
           }

I can't get the list to return. When I break code on "return ObjectList", there are 3 objects in the list, however, on return to the caller, a null value is returned.

Does anyone know what's wrong with this picture?

Hint: You're not using destination, and you're not saving the return value from CopyAllTo. — ProgrammingLlama
– ProgrammingLlama, Commented May 19, 2020 at 6:07
I added a line before the return statement: destination = objectList, following that with return of destination. Not sure if you meant something like this or not by "Saving". I mean the objectList does have the items I'm looking for, but in return, there's nothing there. — Romel Evans
– Romel Evans, Commented May 19, 2020 at 6:17

Cleptus · Accepted Answer · 2020-05-19 06:23:58Z

2

Either you'll have to assign the value to abc in your GetPeople method (remove the destination argument in your CopyAllTo method):

static List<NewPeople> GetPeople()
{
    List<People> ppl = Enttity.GetCollection();
    List<NewPeople> abc = ppl.CopyAllTo<People,NewPeople>();

    return abc;
    //return nppl;
}

Or you use the destination variable in your CopyAllTo method (this is possible because List<T> is a reference type):

public static void CopyAllTo<T, U>(this IEnumerable<T> source, List<U> destination) 
    where T : class 
    where U : class
{
    foreach (T t in source)
    {
        U u = Activator.CreateInstance<U>();
        t.CopyTo(u);
        destination.Add(u);
    }
}

edited May 19, 2020 at 6:23

Cleptus

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answered May 19, 2020 at 6:08

SomeBody

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