c# combobox autocomplete behavior

while (reader.Read()) 
{
	combobox1.Items.Add(reader["SomeFieldName"].ToString());
}

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List<SomeObject> objects = new List<SomeObject>()
while (reader.Read())
{
	objects.Add(new SomeObject() { SomeFieldName = reader["SomeFieldName"].ToString() });
}
combobox1.DataSource = objects;

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try
            {
                SqlConnection read = Cts.Login.ConnectRead();

                SqlCommand mycom = read.CreateCommand();

                mycom.CommandText = "SELECT Selection " +
                     "FROM ItemsQuinc ORDER BY Selection ASC";

                SqlDataAdapter da = new SqlDataAdapter(mycom);

                DataSet dataSet = new DataSet();
                da.Fill(dataSet);

                itemComboBox.DataSource = dataSet.Tables[0];

                itemComboBox.DisplayMember = "Selection";

                read.Close();
                read.Dispose();

            }

            catch (SqlException ex)
            {
                System.Windows.Forms.MessageBox.Show(ex.Message);
                return;
            }

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