i can fill my listdictinary but, if running error returns to me in " foreach (string ky in ld.Keys)"(invalid operation Exception was unhandled)

Error Detail : After creating a pointer to the list of sample collection has been changed. C#

       ListDictionary ld = new ListDictionary();
            foreach (DataColumn dc in dTable.Columns)
            {
                MessageBox.Show(dTable.Rows[0][dc].ToString());
                ld.Add(dc.ColumnName, dTable.Rows[0][dc].ToString());
            }

            foreach (string ky in ld.Keys)
                if (int.TryParse(ld[ky].ToString(), out QuantityInt))
                    ld[ky] = "integer";
                else if(double.TryParse(ld[ky].ToString(), out QuantityDouble))
                    ld[ky]="double";
                else
                    ld[ky]="nvarchar";