Question

This is as close as I’ve gotten to successfully populating an expandable listview from a sqlite table.

public class Today : ExpandableListActivity
{
   protected override void OnCreate(Bundle bundle)
    {
        base.OnCreate(bundle);

       IList<IDictionary<string, object>> parent = new IList<IDictionary<string,object>>();
       IList<IList<IDictionary<string, object>>> child = new IList<IList<IDictionary<string,object>>>();

        External inst = new External();
        var connection = inst.conn();
        var c = connection.CreateCommand();
        c.CommandText = "Select Distinct Store From Calls";
        SqliteDataReader dr = c.ExecuteReader();
        if (dr.HasRows)
            while (dr.Read())
            {
                IDictionary<string, object> pItem = new IDictionary<string,object>();
                pItem.Add("Store", dr[0].ToString());
                parent.Add(pItem);
            }
       dr.Close();
       int cnt = parent.Count();

       if (cnt > 0)
       {
           IList<IDictionary<string, object>> children = new IList<IDictionary<string, object>>();
           foreach(IDictionary<string, object> d in parent)
           {
               c.CommandText = "Select CallNumber From Calls Where Store = '" + d.Values + "'";
               dr = c.ExecuteReader();
               while (dr.Read())
               {
                   IDictionary<string, object> childItem = new IDictionary<string, object>();
                   childItem.Add("Call", dr[0].ToString());
                   children.Add(childItem);
               }
               dr.Close();
           }
           child.Add(children);
       }

       SimpleExpandableListAdapter adapter = new SimpleExpandableListAdapter(this, parent, Android.Resource.Layout.SimpleExpandableListItem1, new string[] { "Store" }, new int[] { Android.Resource.Id.Text1, Android.Resource.Id.Text2 }, child, Android.Resource.Layout.SimpleExpandableListItem2, new string[] { "CallNumber" }, new int[] { Android.Resource.Id.Text1, Android.Resource.Id.Text2 });
       SetListAdapter(adapter);
    }

}

This throws the following exceptions:

• Cannot create an instance of the abstract class or interface ‘System.Collections.Generic.IDictionary<string,object>‘ X2
• Cannot create an instance of the abstract class or interface ‘System.Collections.Generic.IList<System.Collections.Generic.IDictionary X2

These exceptions really don’t tell me why it can’t create those instances or give me any clue as to what I’m doing incorrectly.

Answer 1

Those are build errors, not exceptions. The reason you’re getting them is that you can’t create an instance of an interface in C#. Instead you need to create an instance of an object that implements the interface you need. For example, using the ones in your code:

IDictionary<string, object> pItem = new Dictionary<string,object>();

IList<IDictionary<string, object>> children = new List<IDictionary<string, object>>();

This is valid because Dictionary<TKey, TValue> implements IDictionary<TKey, TValue>, and List<T> implements IList<T>.