Converting a List of Base type to a List of Inherited Type

Question

I would be certain that this question addresses something that would have been brought up in a previous question, but I was unable to find it.

There is a method in a C# class that takes as a parameter a generic List of a Base Class. I need to pass a list of an inherited class and do not know exactly how to do this. I am getting an error in my attempts. Below is sample code to illustrated this:

public class A
{
   public static void MethodC(List<A>)
   {
       // Do Something here with the list
    }
}
public Class B : A
{
   // B inherits from A, A is the Base Class   
}

// Code utilizing the above method 
List<B> listOfB = new List<B>();
A.MethodC( (List<A>) listOfB );  // Error: this does not work
A.MethodC( listOfB.ToList<typeof(A)>() ); // Error: this does not work
A.MethodC( listOfB.ConvertAll<A>(typeof(A)) ); // Error: this does not work
// how can I accomplish this?  It should be possible I would think

Note: Here is my final working Method as a reference. I got an even better solution to my problem, but technically it wasn't an answer to the question, since my question was phrased impropertly.

 public static DataTable 
    ObjectCollectionToDataTable<GLIST>
      (List<GLIST> ObjectCollection) where GLIST 
              : BaseBusinessObject
        {
            DataTable ret = null;

            if (ObjectCollection != null)
            {
                foreach ( var b in ObjectCollection)
                {

                    DataTable dt = b.ToDataTable();
                    if (ret == null)
                        ret = dt.Clone();
                    if (dt.Rows.Count > 0)
                        ret.Rows.Add(dt.Rows[0].ItemArray);
                }
            }

            return ret;
        }

Rosamariarosamond

That's the same answer I gave you... – Development

And it is technically the answer to what you originally asked, as well! :) – Development

Answer 1

If you have linq available you can do

var ListOfA = ListOfB.Cast<A>().ToList();

Tannertannery

I do not think I have linq, but it worked anyway. thank you very much for your quick response. – Rosamariarosamond

Possible problem - does MethodC need to modify the list, so that those changes are visible outside of MethodC? – Development

Glad it worked! The only reason I believe you have the LINQ extension methods available was the use of ToList in your example. – Tannertannery

now, it doesn't need to modify the list, it foreachs through the collection and calls abstract methods on each one. – Rosamariarosamond

You can skip the '.Cast<A>': var ListOfA = ListOfB.ToList<A>(); – Poinsettia

Answer 2

You cannot do that. To understand why it is not allowed, imagine what would happen if Add was called on a List<Derived> after it had been cast to a List<Base>.

Also, the answers implying that C# 4.0 will be different are wrong. List will never be modified to allow you to do this. Only IEnumerable will - because it does not allow items to be added to the collection.

Update: The reason it works in the solution you've gone for is because you're no longer passing the same list. You're creating a whole new list which is a copy of the original. This is why I asked about modifying the list; if MethodC makes changes to the number of items on the list, those changes would be made to a copy, not the original list.

I think the ideal solution for you is as follows:

public abstract class A
{
    public void MethodC<TItem>(List<TItem> list) where TItem : A
    {
        foreach (var item in list)
            item.CanBeCalled();
    }

    public abstract void CanBeCalled();
}

public class B : A
{
    public override void CanBeCalled()
    {
        Console.WriteLine("Calling into B");
    }
}

class Program
{
    static void Main(string[] args)
    {
        List<B> listOfB = new List<B>();

        A a = new B();

        a.MethodC(listOfB);
    }
}

Notice how, with this solution, you can pass a List<B> directly to MethodC without needing to do that weird conversion on it first. So no unnecessary copying.

The reason this works is because we've told MethodC to accept a list of anything that is derived from A, instead of insisting that it must be a list of A.

Development

I don't think that is true, it works fine. My method call works loops through the collection and acts on abstract (override) methods and properties. I used the code above in a previous response, and it works great. the project is .NET 2.0 with CSC.exe (C#) 3.0 as the compiler. – Rosamariarosamond

I'm guessing you're having difficulty getting this to work? :) – Development

Answer 3

You are addressing the lack of covariance in the current C# version. Here is one way of doing it:

listOfB.Cast<A>();

Vulcanology

This only gives you an IEnumerable<A>, not List<A>. – Desist

this works because ((List<A>)listOfB.Cast<A>()) gives me what I want when I explicitly cast it as a List. – Rosamariarosamond

Answer 4

Here is an answer that will exclude any objects in your list of the wrong type. This is a much safer way in my opinion:

List<A> ListOfA  = ListOfB.OfType<A>().ToList();

The OfType method will exclude items of the wrong derived class where as the Cast will throw an error.

Fimbria