C# Excel combobox 中的空行（导致错误“System.FormatException：输入字符串格式不正确。”）

Question

first of all, sorry for my bad english, but i'm a beginner with C# who's trying to make a simple Excel application (a school work) where you can edit Excel cells of player list and add new players in the list. Editing works perfectly, but I can't add new players on the list. When i look at the combobox of the application, there's lots of empty cells, and probably because of that, adding new player isn't working.

When trying to add a new player, it gives a following error:

System.FormatException:Input string was not in a correct format.

When I'm trying to debug the problem, it shows that the problem is in this line:

int nextID = Convert.ToInt32(lastID) + 1; // Lasketaan seuraavan ID:n arvo

Thanks already, and don't hesitate to ask, if you need more information to help me!

Here's the code for the actual program.

    using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Microsoft.Office.Interop.Excel;
 
namespace Luokat.Persons
{
    public class Persons : ExcelInstance
    {
        //variables internal use
        private string id;
        private string lastname;
        private string firstname;
        private string email;
        private string postalcode;
        private string city;
        private string phone;
        private ArrayList arInfo = new ArrayList();
        private string ok = "0";
        private string found = "0";
        private string error = "0";
 
 
        //properties external use
        public string Id
        {
            get
            {
                return id;
            }
            set
            {
                id = value;
            }
        }
        public string Lastname
        {
            get
            {
                return lastname;
            }
            set
            {
                lastname = value;
            }
        }
        public string Firstname
        {
            get
            {
                return firstname;
            }
            set
            {
                firstname = value;
            }
        }
        public string Email
        {
            get
            {
                return email;
            }
            set
            {
                email = value;
            }
        }
        public string Postalcode
        {
            get
            {
                return postalcode;
            }
            set
            {
                postalcode = value;
            }
        }
        public string City
        {
            get
            {
                return city;
            }
            set
            {
                city = value;
            }
        }
        public string Phone
        {
            get
            {
                return phone;
            }
            set
            {
                phone = value;
            }
        }
        public ArrayList ArInfo
        {
            get
            {
                return arInfo;
            }
            set
            {
                arInfo = value;
            }
        }
        public string Ok
        {
            get
            {
                return ok;
            }
            set
            {
                ok = value;
            }
        }
        public string Found
        {
            get
            {
                return found;
            }
            set
            {
                found = value;
            }
        }
        public string Error
        {
            get
            {
                return error;
            }
            set
            {
                error = value;
            }
        }
        //Methods
        public void searchPersons()
        {
            try
            {
                int totalRows = xlRange.Rows.Count;
                int totalColumns = xlRange.Columns.Count;
 
                for (int rowCount = 2; rowCount <= totalRows; rowCount++)
                {
                    this.id = xlRange.Cells[rowCount, 1].Text;
                    this.lastname = xlRange.Cells[rowCount, 2].Text;
                    this.firstname = xlRange.Cells[rowCount, 3].Text;
 
 
                    ArInfo.Add(this.Id + " " + this.Lastname + " " + this.Firstname);
                }
 
                //xlWorkBook.Close(true);
                //xlApp.Quit();
            }
            catch (Exception e)
            {
                this.error = e.ToString();
            }
        }
        public void searchPerson(string sID)
        {
            /*Microsoft.Office.Interop.Excel.Application xlApp = new Microsoft.Office.Interop.Excel.Application();
            Microsoft.Office.Interop.Excel.Workbook xlWorkBook = xlApp.Workbooks.Open(FilePath);
            Microsoft.Office.Interop.Excel.Worksheet xlWorkSheet = (Microsoft.Office.Interop.Excel.Worksheet)xlWorkBook.Worksheets.get_Item(1);
 
            Microsoft.Office.Interop.Excel.Range xlRange = xlWorkSheet.UsedRange;*/
 
            int totalRows = xlRange.Rows.Count;
            int totalColumns = xlRange.Columns.Count;
 
            for (int rowCount = 2; rowCount <= totalRows; rowCount++)
            {
                this.id = xlRange.Cells[rowCount, 1].Text;
                if (this.id == sID)
                {
                    this.Email = xlRange.Cells[rowCount, 4].Text;
                    this.Postalcode = xlRange.Cells[rowCount, 5].Text;
                    this.City = xlRange.Cells[rowCount, 6].Text;
                    this.Phone = xlRange.Cells[rowCount, 7].Text;
                }
            }
        }
        public void editPerson(string sID, string sLastname, string sFirstname, string sEmail, string sPostalcode, string sCity, string sPhone)
        {
 
            int totalRows = xlRange.Rows.Count;
            int totalColumns = xlRange.Columns.Count;
 
            for (int rowCount = 2; rowCount <= totalRows; rowCount++)
            {
                this.id = xlRange.Cells[rowCount, 1].Text;
                if (this.id == sID)
                {
                    xlRange.Cells[rowCount, 2] = sLastname;
                    xlRange.Cells[rowCount, 3] = sFirstname;
                    xlRange.Cells[rowCount, 4] = sEmail;
                    xlRange.Cells[rowCount, 5] = sPostalcode;
                    xlRange.Cells[rowCount, 6] = sCity;
                    xlRange.Cells[rowCount, 7] = sPhone;
                    this.ok = "1";
                    break;
                }
            }
        }
        public void newPerson(string sLastname, string sFirstname, string sEmail, string sPostalcode, string sCity, string sPhone)
        {
 
            int totalRows = xlRange.Rows.Count;
            int totalColumns = xlRange.Columns.Count;
            int nextRow = totalRows + 1; // Lisätään rivien arvoa yhdellä
            string lastID = xlRange.Cells[totalRows, 1].Text; // Haetaan viimeinen ID
            int nextID = Convert.ToInt32(lastID) + 1; // Lasketaan seuraavan ID:n arvo
 
            // Tarkistetaan sähköposti
            for (int rowCount = 2; rowCount <= totalRows; rowCount++)
            {
                this.email = xlRange.Cells[rowCount, 4].Text;
 
                if (this.email == sEmail)
                {
                    this.found = "1"; // Jos löytyi, annetaan muuttujalle arvo 1
                    break;
                }
            }
 
            if (found == "1") // Jos löytyi suljetaan instanssit ja poistutaan
            {
                xlWorkBook.Close(true);
                xlApp.Quit();
                return;
            }
 
            // Jos sähköposia ei löydy, lisätään uusi henkilö tietoineen
            xlRange.Cells[nextRow, 1] = nextID;
            xlRange.Cells[nextRow, 2] = sLastname;
            xlRange.Cells[nextRow, 3] = sFirstname;
            xlRange.Cells[nextRow, 4] = sEmail;
            xlRange.Cells[nextRow, 5] = sPostalcode;
            xlRange.Cells[nextRow, 6] = sCity;
            xlRange.Cells[nextRow, 7] = sPhone;
 
            this.ok = "1";
        }
        public void removePerson(string sID)
        {
 
            int totalRows = xlRange.Rows.Count;
 
            for (int rowCount = 2; rowCount <= totalRows; rowCount++)
            {
                this.id = xlRange.Cells[rowCount, 1].Text;
                if (this.id == sID)
                {
                    xlWorkSheet.Rows[rowCount].EntireRow.Delete();
 
                    this.ok = "1";
                    break;
                }
            }
        }
        public void closeExcel()
        {
            xlWorkBook.Close(true); //true = autosave
            xlApp.Quit();
        }
    }
}

Answer 1

lastID may or may not be int.

Use Int32.TryParse(value, out number) instead. That will solve your problem.

int lastIDValue;
Int32.TryParse(lastID, out lastIDValue)

Answer 2

Try use int.TryParse

if(int.TryParse(yourStr, out yourInt) == true)
{
    //if success
}
else
{
    //if fail
}