如何通过C＃.Net中的ComboBox检索第一个索引的值？

Question

I kept getting "FormatException was unhandled by user Code" Following is the Code:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Data.Sql;
using System.Data.SqlClient;
using System.Configuration;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace Eventmanagement
{
    public partial class Registration : Form
    {
        SqlConnection aConnection;
        string firstname= string.Empty; 
        string lastname= string.Empty;
        int aid;
        string date = DateTime.Now.ToShortDateString();
        SqlDataAdapter da = new SqlDataAdapter();
        DataTable dta;


        public Registration(string fname, string lname, int attID)
        {
            this.firstname = fname;
            this.lastname = lname;
            this.aid = attID;
            InitializeComponent();
        }
        //--------------------------------------------//

        private void Registration_Load(object sender, EventArgs e)
        {
            // TODO: This line of code loads data into the 'insertEventDataSet.Events' table. You can move, or remove it, as needed.

            populateEventSalesPersonList();
            populateEventNameIdTypeList();

            //+++++++++++++++++++++++++++++++++++++++++++//
            txtSalesTaxRate_Registration.Text = Convert.ToString( SalesTaxRate());
            txtRegistrationID_Registration.Text = regID().ToString();
            //+++++++++++++++++++++++++++++++++++++++++++//

            //+++++++++++++++++++++++++++++++++++++++++++//
            txtAttendee_Registration.Text = (this.firstname+" "+this.lastname);
            txtRegistrationDate_Registration.Text = date.ToString();


        }

        //--------------------------------------------//

        public string getConnectionString()
        {
            try
            {
                string sConnection = "";

                // Get the mapped configuration file.
                System.Configuration.ConnectionStringSettingsCollection ConnSettings = ConfigurationManager.ConnectionStrings;
                sConnection = ConnSettings["DBConnectionString"].ToString();

                return sConnection;


            }
            catch (Exception err)
            {
                MessageBox.Show(err.Message);
                return "";
            }
        }



        //--------------------------------------------//
        public void populateEventNameIdTypeList()
        {
            try
            {
                cmbEvent_Registration.DataSource = getDataTable4();
                //----------------------------
                cmbEvent_Registration.ValueMember = "EventID";
                cmbEvent_Registration.DisplayMember = "EventName";




            }
            catch (Exception err)
            {
                MessageBox.Show(err.Message);
            }


        }


        //--------------------------------------------//

        public void populateEventSalesPersonList()
        {
             try
            {

                cmbSalesPerson_Registration.DataSource = getDataTable5();
                cmbSalesPerson_Registration.ValueMember = "EmployeeID";
                cmbSalesPerson_Registration.DisplayMember = "SalesPerson";

            }
            catch (Exception err)
            {
                MessageBox.Show(err.Message);
            }
        }

        //-------------------------------------------//

        public void populateFeeScheduleByEventList()
        {

            try
            {

                cmbFeeSchedule_Registration.DataSource = getDataTable6(); 
                cmbFeeSchedule_Registration.ValueMember = "FeeScheduleID";
                cmbFeeSchedule_Registration.DisplayMember = "Fee";




                //--------------------------------------------------//

                //saleTax();
                //txtSalesTax_Registration.Text = Convert.ToString(saleTax());
                //txtTotalCharges_Registration.Text = Convert.ToString(totalCharges());
                //txtAmountDue_Registration.Text = Convert.ToString(amountDue());

                //--------------------------------------------------//
                }

            catch (Exception err)
            {
                MessageBox.Show(err.Message);
            }

        }

        //------------------------------------------//

        //------------------------------------------//



        private void btnclose_Registration_Click(object sender, EventArgs e)
        {
            this.Close();
        }

        private void btnInsert_Registration_Click(object sender, EventArgs e)
        {
            try
            {
                aConnection = new SqlConnection(getConnectionString());
                aConnection.Open();

                //Calling the Stored Procedure

                da.InsertCommand = new SqlCommand("RegistrationInsert", aConnection);
                da.InsertCommand.CommandType = CommandType.StoredProcedure;

                //da.InsertCommand.Parameters.Add(@"RegistrationID", SqlDbType.Int).Value =  Convert.ToInt16( txtRegistrationID_Registration.Text);
                da.InsertCommand.Parameters.Add(@"AttendeeID", SqlDbType.Int).Value =  Convert.ToInt16( aid); 
                da.InsertCommand.Parameters.Add(@"RegistrationDate", SqlDbType.SmallDateTime).Value = Convert.ToDateTime(txtRegistrationDate_Registration.Text=date.ToString());
                da.InsertCommand.Parameters.Add(@"PurchaseOrderNumber", SqlDbType.VarChar).Value = txtPoNumber_Registration.Text; 
               // da.InsertCommand.Parameters.Add(@"SalesPerson", SqlDbType.Int).Value = Convert.ToInt64(cmbSalesPerson_Registration.SelectedValue.ToString());
                da.InsertCommand.Parameters.Add(@"EventID", SqlDbType.Int).Value = cmbEvent_Registration.SelectedValue.ToString();
                da.InsertCommand.Parameters.Add(@"FeeScheduleID", SqlDbType.Int).Value = cmbFeeSchedule_Registration.SelectedValue.ToString();
                da.InsertCommand.Parameters.Add(@"RegistrationFee", SqlDbType.Money).Value = txtRegistrationFee_Registration.Text;
                da.InsertCommand.Parameters.Add(@"EmployeeID", SqlDbType.Int).Value = Convert.ToInt16(cmbSalesPerson_Registration.SelectedValue.ToString());
                da.InsertCommand.Parameters.Add(@"SalesTaxRate", SqlDbType.Float).Value = txtSalesTaxRate_Registration.Text;
                //da.InsertCommand.Parameters.Add(@"EndTime", SqlDbType.SmallDateTime).Value = Convert.ToDateTime(txtEndTime.Text.ToString());
                da.InsertCommand.ExecuteNonQuery();


                MessageBox.Show("Inserted successfully!!!");

                aConnection.Close();
                da.InsertCommand.Dispose();

            }
            catch (Exception err)
            {
                MessageBox.Show(err.Message);
            }


        }

        //-------------------------------------------//

        public DataTable getDataTable4()
        {
            try
            {
                dta = new DataTable();
                aConnection = new SqlConnection(getConnectionString());
                aConnection.Open();
                da = new SqlDataAdapter();
                da.SelectCommand = new SqlCommand("EventsSelectAll", aConnection);
                da.SelectCommand.CommandType = CommandType.StoredProcedure;
                da.SelectCommand.ExecuteNonQuery();

                da.Fill(dta);
                aConnection.Close();
                return dta;

            }
            catch (Exception err)
            {
                MessageBox.Show(err.Message);
                return null;
            }
        }
        public DataTable getDataTable5()
        {
            try
            {
                dta = new DataTable();
                aConnection = new SqlConnection(getConnectionString());
                aConnection.Open();
                da = new SqlDataAdapter();
                da.SelectCommand = new SqlCommand("sp_RegistrationSalesPerson", aConnection);
                da.SelectCommand.CommandType = CommandType.StoredProcedure;
                da.SelectCommand.ExecuteNonQuery();
                dta.Clear();
                da.Fill(dta);
                aConnection.Close();
                return dta;

            }
            catch (Exception err)
            {
                MessageBox.Show(err.Message);
                return null;
            }
        }
        public DataTable getDataTable6()
        {
            try
            {
                dta = new DataTable();
                da = new SqlDataAdapter();
                aConnection = new SqlConnection(getConnectionString());

                aConnection.Open();

                da.SelectCommand = new SqlCommand("sp_FeeScheduleSelectAllByEventID", aConnection);
                da.SelectCommand.Parameters.Add("EventID", SqlDbType.Int).Value = Convert.ToInt32(cmbEvent_Registration.SelectedValue.ToString());
                da.SelectCommand.CommandType = CommandType.StoredProcedure;


                da.SelectCommand.ExecuteNonQuery();
                da.Fill(dta);
                aConnection.Close();
                return dta;
            }
            catch (Exception err)
            {
                MessageBox.Show(err.Message);
                return null;
            }


        }



        private void cmbEvent_Registration_SelectedIndexChanged(object sender, EventArgs e)
        {
                populateFeeScheduleByEventList();

                txtRemainingSeats_Registration.Text = Convert.ToString(spaces());
        }




        public double saleTax()
        {                               
            double regFee = Convert.ToDouble(cmbFeeSchedule_Registration.SelectedText);
            double sTR = Convert.ToDouble(SalesTaxRate());
            double sr = regFee * (sTR / 100);
            return sr;
        }
        public int totalRegisteredAttendees()
        {
            da = new SqlDataAdapter();
            aConnection = new SqlConnection(getConnectionString());
            da.SelectCommand = new SqlCommand("RegistrationSelectAllByEventID", aConnection);
            da.SelectCommand.CommandType = CommandType.StoredProcedure;
            da.SelectCommand.Parameters.Add("EventID", SqlDbType.Int).Value = Convert.ToInt32(cmbEvent_Registration.SelectedValue.ToString());

            aConnection.Open();
            int val = (int)da.SelectCommand.ExecuteScalar();
            aConnection.Close();
            return val;
        }
        public int spaces()
        {
            da = new SqlDataAdapter();
            aConnection = new SqlConnection(getConnectionString());
            da.SelectCommand = new SqlCommand("EventsSelect", aConnection);
            da.SelectCommand.CommandType = CommandType.StoredProcedure;
            da.SelectCommand.Parameters.Add("EventID", SqlDbType.Int).Value = Convert.ToInt32(cmbEvent_Registration.SelectedValue.ToString());
            aConnection.Open();

            int spaces = Convert.ToInt16(da.SelectCommand.ExecuteScalar());
            aConnection.Close();

            int value = totalRegisteredAttendees();
            int totalspaces = spaces - value;
            return totalspaces;

        }

        public double SalesTaxRate()
        {
            da = new SqlDataAdapter();
            aConnection = new SqlConnection(getConnectionString());
            da.SelectCommand = new SqlCommand("CompanyInfo", aConnection);
            da.SelectCommand.CommandType = CommandType.StoredProcedure;

            aConnection.Open();
            double sale = Convert.ToDouble(da.SelectCommand.ExecuteScalar());
            aConnection.Close();
            return sale;
        }


        public double totalCharges()
        {
            double tc = Convert.ToDouble(txtRegistrationFee_Registration.Text) +  (saleTax());
            return tc;
        }
        public double amountDue()
        {
            double aD;
            if (txtTotalPaid_Registration.Text == string.Empty)
            {
                aD = Convert.ToDouble(txtTotalCharges_Registration.Text.ToString());
            }
            else
            {
                double a = Convert.ToDouble(txtTotalPaid_Registration.Text.ToString());
                 double b=    (Convert.ToDouble(txtTotalCharges_Registration.Text.ToString()));
                 aD = b-a;
            }
                return aD;
        }

        public int regID()
        {
            da = new SqlDataAdapter();
            aConnection = new SqlConnection(getConnectionString());
            da.SelectCommand = new SqlCommand("RegistrationID", aConnection);
            da.SelectCommand.CommandType = CommandType.StoredProcedure;

            aConnection.Open();
            int id = ((int)da.SelectCommand.ExecuteScalar())+1;

            aConnection.Close();
            return id;  
        }

        private void cmbFeeSchedule_Registration_SelectedIndexChanged(object sender, EventArgs e)
        {
           saleTax();
        }

        //------------------------------------------//
    }
}

When I call SaleTax() Method in following

private void cmbFeeSchedule_Registration_SelectedIndexChanged(object sender, EventArgs e)
        {
           saleTax();
        }

I get "FormatException was unhandled by user Code" Error I debugged the Code and find out that it is happening because cmbFeeSchedule_Registration is not selecting the first index, hence it is giving the error. I am confused what to do? How I get around this Problem?

Edit: Well I resolved the issue. It was the problem of in ` public void populateFeeScheduleByEventList() {

        try
        {

            cmbFeeSchedule_Registration.DataSource = getDataTable6(); 
            cmbFeeSchedule_Registration.ValueMember = "FeeScheduleID";
            cmbFeeSchedule_Registration.DisplayMember = "Fee";
        catch (Exception err)
        {
            MessageBox.Show(err.Message);
        }

    }`  

When SaleTax() was called, pointer leaves the DisplayMember and ValueMember . Hence I readjusted the code little bit and placed cmbFeeSchedule_Registration.ValueMember = "FeeScheduleID"; cmbFeeSchedule_Registration.DisplayMember = "Fee"; before cmbFeeSchedule_Registration.DataSource = getDataTable6(); and it resolved the Problem. :)

Answer 1

If it is required that index 0 be selected in your ComboBox then you need to do some basic checking. In your SelectedIndexChanged EventHandler wire-up, you need to add:

if (cmbFeeSchedule_Registration.SelectedIndex == 0) {
    salesTax();
}

Also if the user is able to change the text in the ComboBox, you are not doing any error handling to handle the FormatException that will be caused by Convert.ToDouble when trying to convert a non-double value that is not parsable.

double regFee = Convert.ToDouble(cmbFeeSchedule_Registration.SelectedText);            
double sTR = Convert.ToDouble(SalesTaxRate());

Furthermore, you could actually use double.Parse or double.TryParse instead of ConvertTo, which seems more proper. On another note, if the user cannot edit the text in the ComboBox (say you have it set to DropDownList), you should use the SelectedObject or SelectedValue property instead of SelectedText depending if it is DataBound or not.

Bottom line is you need to make sure you are checking that the correct index is selected before calling salesTax() or any operation that will potentially throw an exception, and do error checking to ensure that your values are what they should be. If regFee is anything but a double, because the correct index is not selected, ConvertTo will fail. (Use double.Parse/TryParse instead, anyway)

A tip of advice when posting questions is to only post the affected segment of code, and provide a StackTrace when one is available. It's too difficult to navigate all of your code on here, and using a StackTrace it is easy to break down.

Answer 2

你必须在表格中使selectedindex = 0

Answer 3

Just do this:

if(((ComboBox)sender).SelectedIndex > -1) saleTax();

instead. If you want to select the first item when the form loads, set SelectedIndex in the Load event.