Adding to a string list XMLArrayItem using Get Set

Hey guys I'm having a problem with a List Get Set on C# this is what I have

Anyone Please help me see where the problem is

The problem occurs on the get trying to add the selectedvalue

I'm thinking I wrote the get set wrong but I don't see it. I'm not used get sets with lists.

[Serializable]

public class ProcessObjectParts

{

///

/// Object Part Lists

///

[XmlArrayItem("Parts")]

public List Parts { get; set; }

}

and the Get set goes like this:

get {

ProcessObjectParts parts = new ProcessObjectParts();

parts.Parts.Add(DropDownControl.SelectedValue);

parts.Parts.Add(DropDownControl.SelectedItem.Text);

return parts;

}

set {

var parts = GlobalHelper.DeserializeData(value.ToString());

DropDownControl.SelectedValue = parts.Parts[0];

DropDownControl.SelectedItem.Text = parts.Parts[1];

ViewState[parts.Parts[0]] = value;

}