TList and Creating New Class that is a List of Class of my own is updating Tlist

Question

I have a WizardInfo class which as several TLists as properties, this then populates as the user goes through the wizard on the last screen I query the Tlists and make them into Lists and private fields I then create Lists of DefaultItems from these lists. This is my own class and as name and Id as its property. He is some code

public class DefaultItem
{
   public int ID {get;set;}
   public string Name {get;set;}
} 

private List<DefaultItem> _defaultList = null;
_defaultList = new List<DefaultItem>();
defaultValue = PopulateDefaultList(_asmgps, defaultList);
private int PopulateDefaultList(
        List<ASGMP> asmgps,
        ref List<DefaultItem> defaultList)
{
   int isdefault = -1;
   foreach (ASGMP asgmp in asgmps)
   {
      if (asgmp.IsChecked)
      {
         if (asgmp.IsDefault)
         {
            isdefault = asgmp.ID;
         }

         DefaultItem defaultItem = new DefaultItem();
         defaultItem.ID = asgmp.ID;
         defaultItem.Name = GetMPTName(asgmp.ID);  
         defaultList.Add(defaultItem);
      }
   }

   return isdefault;
}

private string GetMPTName(int ID)
{
    try
    {
        SGMP sgmp = DataRepository.SGMPProvider.GetByASGMPID(ID)
        if (serviceGroupMailPresentation != null)
        {
            MPT mpt DataRepository.MPTProvider.GetByMPTID(SGMP.MPTID);
            if (mailPresentationType != null)
            {
                return mpt.Name;
            }
        }

        return string.Empty;
    }
    catch (Exception ex)
    {
        WindowsEventLog.Write(ex);
        throw;
    }
}

The problem i am having is when i remove a item from the defaultList it affects asgmp. I have found the answer. When I get the mpt name I get asgmp from the database this is where Codesmith does a strange thing and connects the usage of the List and the DefaultList. By querying the original List instead of going to the database it now works fine.

Answer 1

I found out that this is because of ntiers instead of using the database the to get the ID I should of used the in List of T in

Answer 2

It is being removed because List<T> is derived from object, and is a Reference type. Reference types are passed by reference, i.e. when you pass your list, you are passing a pointer to its location in memory. So any changed you make on the copied reference, will also be reflected on the original object.

In order to make a copy you can change this like: defaultValue = PopulateDefaultList(_asmgps, defaultList);

to this:

defaultValue = PopulateDefaultList(_asmgps.ToList(), defaultList);

This will enumerate the collection as IEnumerable<T> and return is as a list. This will effectivlly create a copy.

Answer 3

erm, instead of PopulateDefaultList why not just do,

var defaultList = asgmps
        .Where(asgmp => asgmp.IsChecked)
        .Select(asgmp => new
            {
                IsDefault = asgmp.IsDefault,
                Item =  new DefaultItem
                    {
                        ID = asgmp.ID,
                        Name = GetMPTName(asgmp.ID)
                    }
            }).ToList();

of course, naming a collection defaultList that contains non-defaults seems counter intuitive.