</h3>
        <asp:Label ID="Label1" runat="server" Text="Марка  :"></asp:Label>
 
        <asp:DropDownList ID="DDL_Brands" runat="server" 
            DataSourceID="ObjectDataSource1" Height="25px" Width="182px" 
            DataTextField="brandOfCar" DataValueField="brandOfCar">
            <asp:ListItem></asp:ListItem>
        </asp:DropDownList>
        <asp:ObjectDataSource ID="ObjectDataSource1" runat="server" 
            SelectMethod="GetBrands_DDL" TypeName="AutoCatalog.InsertDB" 
            onselected="ObjectDataSource1_Selected">
        </asp:ObjectDataSource>
        <br />
        Модель  : 
        <asp:DropDownList ID="DDL_Models" runat="server" 
            DataSourceID="ObjectDataSource2" DataTextField="BrandOfCar" 
            DataValueField="BrandOfCar" Height="19px" Width="181px">
        </asp:DropDownList>
         <asp:ObjectDataSource ID="ObjectDataSource2" 
            runat="server" SelectMethod="GetModels_DDL" TypeName="AutoCatalog.InsertDB">
            <SelectParameters>
                <asp:ControlParameter ControlID="DDL_Brands" Name="brandOfCar" 
                    PropertyName="SelectedValue" Type="String" />
            </SelectParameters>
        </asp:ObjectDataSource>

public class InsertDB
    {
        List<InsertDetails> ListModels;
        List<InsertDetails> ListBrands;    
   
            public List<InsertDetails> GetBrands_DDL()
            {
                SqlConnection connection = Connection.Create();
                SqlCommand cmd = new SqlCommand("GetBrands_DDL", connection);
                cmd.CommandType = CommandType.StoredProcedure;
                ListBrands = new List<InsertDetails>();

                try
                {
                    connection.Open();
                    SqlDataReader reader = cmd.ExecuteReader();
                    while (reader.Read())
                    {
                        InsertDetails insertDetails = new InsertDetails((string)reader["brandOfCar"]);
                        ListBrands.Add(insertDetails);
                    }

                    reader.Close();
                    return ListBrands;
                }
                catch (SqlException err)
                {
                    throw new ApplicationException("Data error Get Brands_DDL.");
                }
                finally
                {
                    connection.Close();
                }
            }


            public List<InsertDetails> GetModels_DDL(string brandOfCar)
            {
                SqlConnection connection = Connection.Create();
                SqlCommand cmd = new SqlCommand("GetModels_DDL", connection);
                cmd.CommandType = CommandType.StoredProcedure;
                ListModels = new List<InsertDetails>();

                cmd.Parameters.Add(new System.Data.SqlClient.SqlParameter("brandOfCar", System.Data.SqlDbType.NVarChar, 30));
                cmd.Parameters["brandOfCar"].Value = brandOfCar;

                try
                {
                    connection.Open();
                    SqlDataReader reader = cmd.ExecuteReader();
                    while (reader.Read())
                    {
                        InsertDetails insertDetails = new InsertDetails((string)reader["nameModel"]);
                        ListModels.Add(insertDetails);
                    }

                    reader.Close();
                    return ListModels;
                }
                catch (SqlException err)
                {
                    throw new ApplicationException("Data error Get Models_DDL.");
                }
                finally
                {
                    connection.Close();
                }
           }
    }