Question

Possible Duplicate:
In C#, why can't a List<string> object be stored in a List<object> variable

I am having the following code.

   public class Manufacturer :  IHierarchicalEntity
    {
        public string ManufacturerName
        {
            get
            {
                return _manfuacturerName;
            }
            set
            {
                _manfuacturerName = value;
            }
        } private string _manfuacturerName;

        public List<Product> Products
        {
            get
            {
                return _products;
            }
        } private List<Product> _products;

        #region IHierarchicalEntity Members

        public List<IHierarchicalEntity> Children
        {
            get
            {
                return Products; //This is where I get the compiler error
            }
        }

        #endregion
    }

    public class Product :  IHierarchicalEntity{}

    public interface IHierarchicalEntity
    {
        List<IHierarchicalEntity> Children { get; }
    }

I get a compiler exception that

Cannot implicitly convert type System.Collections.Generic.List<Library.Product> to System.Collections.Generic.List<Library.IHierarchicalEntity>

Both Manufacturer and Product are of type IHierarchicalEntity. Why is it not taking the List<Product> as List<IHierarchicalEntity> ?

Answer 1

This conversion is not possible, otherwise you could add a OtherHierarchicalEntity to List<Product> so its not safe. You can cast explicitly and return a new list:

  return Products.Cast<IHierarchicalEntity>().ToList();