c# delete excel sheets using for loop

Keep in mind that while deleting a worksheet, index position might get change so you'll also need to decrease the int value to get correct index otherwise you'll again get the same exception while deleting the sheets afterwards or lead to delete a sheet which you dont want to delete

int index = DropDownList1.SelectedIndex;
int max = DropDownList1.Items.Count;

for (int i = 0; i < max; i++)
{
    if (i != index)
    {
        Excel.Worksheet worksheet = (Excel.Worksheet)xlWorkBook.Sheets[i];
        worksheet.Delete();

        //decrease the value
        --i;
    }
}

I don't know but anyhow when I reversed the loop, It started working.

for (i=max; i > 0; i--)
{
    if (i != index+1)
    {
         Excel.Worksheet worksheets = (Excel.Worksheet)xlWorkBook.Sheets[i];
         worksheets.Delete();
    }

}

Lets index=5;
The error means that the sheet at index 5 is not available.
You are correct that the index of worksheet starts with 1 and not 0.

I have done some modification in your code. I am performing an additional check of your current index (i) with the Sheets count present in the workbook to skip the Bad Index Error. Hope this will work.

 using System;
 using Excel = Microsoft.Office.Interop.Excel;
 public partial class WebForm1 : System.Web.UI.Page
 {
     protected void Page_Load(object sender, EventArgs e)
     {     
         int i = 3, max = 5, index = 1;
         string filePath = @"C:\Users\anandra\Desktop\Book1.xlsx";
         Excel.Application excelApp = new Excel.Application();
         Excel.Workbook workBook = excelApp.Workbooks.Open(filePath);
         for (i = 1; i <= max; i++)
         {
           //Adding an extra check here to skip your error
             if (i != index + 1 && workBook.Sheets.Count>i )
             {
                 Excel.Worksheet worksheets = (Excel.Worksheet)workBook.Sheets[i];
                 excelApp.DisplayAlerts = false;
                 worksheets.Delete();
                 excelApp.DisplayAlerts = true;
                 //Decreasing the value of index and i as after deleting the sheet the index will start agarin from 1.
                 i--;
                 index--;
             }
         }
     }
 }