如何在C#.Net中通过ComboBox检索第一个索引的值？

我不断得到“用户代码未处理FormatException” 代码如下：

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();
        }

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

当我在下面调用SaleTax（）方法时

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

I get“用户代码未处理FormatException”错误 我调试了代码，发现这是因为cmbFeeSchedule\u Registration没有选择第一个索引，因此给出了错误。我不知道该怎么办？ 我如何避开这个问题

编辑： 我解决了这个问题。这是“public void populateFeeScheduleByEventList（）中的问题” {

        try
        {

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

    }`  

调用

SaleTax（）

时，指针离开

DisplayMember和ValueMember

。因此我重新调整了代码，并将

cmbFeeSchedule\u Registration.ValueMember=“FeeScheduleID”

cmbFeeSchedule\u Registration.DisplayMember=“Fee”

放在前面

---

cmbFeeSchedule\u Registration.DataSource=getDataTable6（）；

解决了问题。

您必须在表单中设置selectedindex=0

如果需要在组合框中选择索引0，则需要进行一些基本检查。在SelectedIndexChanged EventHandler连接中，您需要添加：

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

此外，如果用户能够更改组合框中的文本，则在尝试转换不可解析的非双精度值时，您不会执行任何错误处理来处理Convert.ToDouble引起的FormatException

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

此外，您实际上可以使用或而不是ConvertTo，这似乎更合适。另一方面，如果用户无法编辑组合框中的文本（假设您将其设置为DropDownList），则应使用SelectedObject或SelectedValue属性而不是SelectedText，具体取决于它是否为数据绑定

底线是，在调用salesTax（）之前，您需要确保选中了正确的索引或者任何可能引发异常的操作，并执行错误检查以确保您的值是正确的值。如果regFee不是double，因为未选择正确的索引，则ConvertTo将失败。（无论如何，请使用double.Parse/TryParse）

发布问题时的一个建议是只发布受影响的代码段，并在有可用代码段时提供StackTrace。在此处导航所有代码太困难，使用StackTrace很容易分解。只需执行以下操作：

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

---

相反。如果要在加载表单时选择第一项，请在

Load

事件中设置

SelectedIndex

。

是否可以发布堆栈跟踪？