increment of row data bound field

Hello Sir,

In my project inside a grid view one textbox value i want to increse its value when rowcommand button add(+) click ed,Sir Please give me the solution urgent sir.

i am using this code 

 

protected void GridView1_RowCommand(object sender, GridViewCommandEventArgs e)
{

DataTable dt = (DataTable)Session["trndata"];
int i = 1;
int rowindex = int.Parse(e.CommandArgument.ToString());
((TextBox)(GridView1.Rows[rowindex].FindControl("txtquantity"))).Text = i.ToString();

if (e.CommandName == "Add")
{
i++;
((TextBox)(GridView1.Rows[rowindex].FindControl("txtquantity"))).Text = i.ToString();
CountTotal();

}