错误：索引超出范围

Question

protected void btnDownload_Click(object sender, EventArgs e)
    {

        //to request the name of the event from the listbox from Main.aspx
        string EventName = Request.QueryString["ename"];

        //Select event id statement
        //const string S = "SELECT EventName FROM Event WHERE EventID = @EventID";
        const string q = "SELECT EventID from Event WHERE EventName = @EventName"; 
        string eventid = "";
        using (SqlConnection c = new SqlConnection(@"Data Source=localhost\SQLEXPRESS;Initial Catalog=PSeminar;Integrated Security=true;Trusted_Connection=Yes;MultipleActiveResultSets=true"))
        using (SqlCommand Command = new SqlCommand(q, c))
        {
            Command.Parameters.AddWithValue("@EventName", EventName);

            c.Open();
            using (SqlDataReader rdr = Command.ExecuteReader())
                while (rdr.Read())
                {
                    Command.CommandText = "Select * from Attendance where EventID=@EventID";
                    System.Text.StringBuilder sb = new System.Text.StringBuilder();
                    sb.Append(String.Format("\"{0}\",\"{1}\", \"{2}\", \"{3}\", \"{4}\", \"{5}\", \"{6}\", \"{7}\"n",
               rdr[0], rdr[1], rdr[2], rdr[3], rdr[4], rdr[5], rdr[6], rdr[7]));
               // I have an error here(Index out of bound)

                    // to get event id from the Event name
                    eventid = rdr.GetString(0);
                    rdr.Close();
                    c.Close();

                    byte[] ar = System.Text.Encoding.UTF8.GetBytes(sb.ToString());
                    Response.ClearContent();
                    Response.ClearHeaders();
                    Response.AddHeader("Content.Type", "application/octet-stream");
                    Response.AddHeader("Content-Length", ar.Length.ToString());
                    Response.AddHeader("Content-Disposition", "attachment; filename=download.csv");
                    Response.BinaryWrite(ar);
                    Response.Flush();
                    Response.End();
                }

        }

The error was - "Index was outside the bounds of the array." I'm trying to download the file according to the event. so far i have done this much of codes. but i do not understand what the error means. pls explain to me what the error "Index was outside the bounds of the array means" and pls give me solutions. thanks

Answer 1

you try to access up to 7 columns in that line, but you only have 1 column (EventId).

Edit:

You can't change the commandtext of a command while reading it. Well, apparently you can, but you won't get the expected results.

Answer 2

The reader contains the result of the following statement

SELECT EventID from Event WHERE EventName = @EventName

and not for this statement

   Select * from Attendance where EventID=@EventID

I would replace const string q with

 const string q = @" Select * from dbo.Attendance 
                  where EventID = (SELECT EventID from dbo.Event WHERE EventName = @EventName");

And instead of * I would use the column names, for two reasons: the database server will like it more and you will be sure which columns you'll have

Answer 3

This error could happen if you read an array and ask for an index greater or equal than the lenght of the array. Checks if the table you're reading has 8 fields or if you're selecting 8 fields.

Answer 4

Index out of bounds means exactly that, you are trying to access somewhere that is past the end of the array.

eg:

var array = new[] {0, 1, 2};
var temp = array[10];

will throw an index out of bounds exception because there is no item at position 10 in array (it only has 3 items and so positions 0, 1 & 2).

This should be enough for you to try solving your problem. If you are still stuck let me know and I'll take a look at your code.