How do I solve get wrong 'input string was not in a correct format.'?

Question

My apsx

       <section class="sec1" style="height:100vh;">
            <link href="../Css/masterStyle.css" rel="stylesheet" />
            <link href="../Css/cartStyle.css" rel="stylesheet" />
        <h1>Cart</h1>
        <p class="sec1_p1">Your Food</p>
        <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" EnableTheming="True"  ShowFooter="True" OnRowDeleting="GridView1_RowDeleting" >
            <Columns>
                <asp:BoundField DataField="sno" HeaderText="sno" Visible="False" />
                <asp:BoundField DataField="restaurantID" HeaderText="restaurantID" Visible="False" />
                <asp:BoundField DataField="foodID" HeaderText="foodID" Visible="False">
                <ItemStyle HorizontalAlign="Center" />
                </asp:BoundField>
                <asp:BoundField DataField="foodName" HeaderText="Name">
                <ItemStyle HorizontalAlign="Center" />
                </asp:BoundField>
                <asp:BoundField DataField="foodPrice" HeaderText="Price">
                <ItemStyle HorizontalAlign="Center" />
                </asp:BoundField>
                <asp:BoundField DataField="quantity" HeaderText="Quantity">
                <ItemStyle HorizontalAlign="Center" />
                </asp:BoundField>
                <asp:BoundField DataField="totalPrice" HeaderText="Total ">
                <ItemStyle HorizontalAlign="Center" />
                </asp:BoundField>
                <asp:CommandField DeleteText="Remove" ShowDeleteButton="True" />
            </Columns>
            <HeaderStyle BackColor="#52E770" ForeColor="White" />
            </asp:GridView>
            <asp:Label ID="test" runat="server" Text="Label"></asp:Label>
        </section>

My .cs

protected void GridView1_RowDeleting(object sender, GridViewDeleteEventArgs e)
    {
        DataTable dt = new DataTable();
        dt = (DataTable)Session["buyitems"];

        for (int i = 0; i <= dt.Rows.Count - 1; i++)
        {
            int sr;
            int sr1;
            string qdata;
            string dtdata;
            sr = Convert.ToInt32(dt.Rows[i]["sno"].ToString());
            TableCell cell = GridView1.Rows[e.RowIndex].Cells[0];
            qdata = cell.Text;
            dtdata = sr.ToString();
            sr1 = Int32.Parse(qdata); //fixed

            if (sr == sr1)
            {
                dt.Rows[i].Delete();
                dt.AcceptChanges();
                //Label1.Text = "Item Has Been Deleted From Shopping Cart";
                break;

            }
        }

        for (int i = 1; i <= dt.Rows.Count; i++)
        {
            dt.Rows[i - 1]["sno"] = i;
            dt.AcceptChanges();
        }

        Session["buyitems"] = dt;
        Response.Redirect("AddToCart.aspx");
    }

I have use Add Wacth in Visual Studio, and i get this result
1.$exception {"Input string was not in a correct format."} System.FormatException
2.qdata = cell.Text This expression causes side effects and will not be evaluated
3.TableCell cell = GridView1.Rows[e.RowIndex].Cells[0]; error CS1073: Unexpected token 'cell'

Answer 1

This error means your string is not in a valid int format.

As pointed by Dai, you need to use TryParse:

var value = "test";
int number;
if (Int32.TryParse(value, out number))
{
   Console.WriteLine("Converted '{0}' to {1}.", value, number);         
}
else
{
   Console.WriteLine("Attempted conversion of '{0}' failed.", 
                      value ?? "<null>");
}

You can also refer to Microsoft documentation here to have more details

Answer 2

You can get rid of the error by using Int32.TryParse

if(!Int32.TryParse(qdata, out sr1)){
   MessageBox.Show(qdata + "is invalid number");
}

Answer 3

Instead of sr1 = Int32.Parse(qdata);

use TryParse as below (and you can combine condition in single if):

if (int.TryParse(qdata, out sr1) && sr == sr1)
{
     dt.Rows[i].Delete();
     dt.AcceptChanges();
     //Label1.Text = "Item Has Been Deleted From Shopping Cart";
     break;
}

UPDATE: Based on aspx code, I believe you are trying to read sno using the code:

TableCell cell = GridView1.Rows[e.RowIndex].Cells[0];

But since that column's (or cell's) visible property is set to false, you will need to handle it differently. One of the ways is using DataKeyNames Property of GridView as shown below:

<asp:GridView ID="GridView1" 
              runat="server" 
              AutoGenerateColumns="False" 
              EnableTheming="True"  
              ShowFooter="True" 
              OnRowDeleting="GridView1_RowDeleting" 
              DataKeyNames = "sno" >

and in code behind, please remove the lines

TableCell cell = GridView1.Rows[e.RowIndex].Cells[0];
qdata = cell.Text;

and directly replace with below condition :

if (int.TryParse(GridView1.DataKeys[e.RowIndex].Value, out sr1) && sr == sr1)
{
     // code to hanlde this case goes here
}

or since you know sno is int, you could directly cast to int. But it throws excpetion if its not int.

 int sno = (int) GridView1.DataKeys[e.RowIndex].Value; // since Value is [object datatype][1] 

If you want to use Cells property, then an option is update the visiable property of "sno" on GridView Row Creation event. If you want to go with this option, you need to remove visible=false from asp:BoundField definition as you will be setting it dynamically in Row Creation event.