Object must implement IConvertible

Question

Object must implement IConvertible.
Description: An unhandled exception occurred during the execution of the current web request.

Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.InvalidCastException: Object must implement IConvertible.

Error Detail Line 279:
da.InsertCommand.Parameters.Add("@Total", SqlDbType.Int).Value = txttotal.Text; Line 280: con1.Open(); Line 281:
da.InsertCommand.ExecuteNonQuery();

Code:

SqlDataAdapter da = new SqlDataAdapter();
    da.InsertCommand = new SqlCommand("Insert Into customer_order(ProductID,product_Name,Product_Type,Weight,Unit_Price,No_Of_Master_Pack,Master_Pack_Price,Quantity,Total)VALUES(@ProductID,@product_Name,@Product_Type,@Weight,@Unit_Price,@No_Of_Master_Pack,@Master_Pack_Price,@Quantity,@Total)", con1);
    da.InsertCommand.Parameters.Add("@ProductID",SqlDbType.VarChar).Value=DropDownList3.SelectedItem;
    da.InsertCommand.Parameters.Add("@product_Name",SqlDbType.VarChar).Value=DropDownList2.SelectedItem;
    da.InsertCommand.Parameters.Add("@Product_Type", SqlDbType.VarChar).Value = DropDownList1.SelectedItem;
    da.InsertCommand.Parameters.Add("@Weight",SqlDbType.Int).Value=txtwgt.Text;
    da.InsertCommand.Parameters.Add("@Unit_Price",SqlDbType.Int).Value=txtmrpsinglepack.Text;
    da.InsertCommand.Parameters.Add("@No_Of_Master_Pack",SqlDbType.Int).Value=txtnoofmasterpack.Text;
    da.InsertCommand.Parameters.Add("@Master_Pack_Price",SqlDbType.Int).Value=txtmrpmaster.Text;
    da.InsertCommand.Parameters.Add("@Quantity",SqlDbType.Int).Value=txtquantity.Text;
    da.InsertCommand.Parameters.Add("@Total", SqlDbType.Int).Value = txttotal.Text;
con1.Open();
da.InsertCommand.ExecuteNonQuery();

Answer 1

ListControl.SelectedItem returns a ListItem - and it doesn't make much sense for the value of a parameter to be a list item itself, but instead the item's value or text. You probably want something like:

da.InsertCommand.Parameters
  .Add("@ProductID",SqlDbType.VarChar).Value = DropDownList3.SelectedItem.Text;

or

da.InsertCommand.Parameters
  .Add("@ProductID",SqlDbType.VarChar).Value = DropDownList3.SelectedItem.Value;