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How to select a row from grid view and edit the details in a new webform using ASP.NET
<asp:HyperLinkField Text="View Details" DataNavigateUrlFields="YourId" DataNavigateUrlFormatString="details.aspx?id={0}" />

if(Request.QueryString["id"])!=null)
  {
      // Fetch respective item from database here & populate form fields
        // Open connection(); execute datareader;populate form fields
   }

<asp:GridView ID="GV_YourGridView" runat="server" OnRowDataBound="gdView_RowDataBound" >
</asp:GridView>

gdView.RowDataBound += new GridViewRowEventHandler(gdView_RowDataBound);

protected void gdView_RowDataBound(object sender, GridViewRowEventArgs e)
    {
        try
        {
            string value = e.Row.Cells[0].Text; //gets your Primary Key from cell on Databind
            e.Row.Cells[0].Controls.Clear(); //Clears existing controls. (not necessary)
            e.Row.Cells[0].Text = "";//Clears the innertext of the cell

            LinkButton LB = new LinkButton();//creates link button
            LB.Text = value;//names it with your primarykey
            LB.Attributes.Add("href", "view-edit.aspx?id=" + LB.Text);// links it to your edit page aswell as passing the primary key
            LB.Attributes.Add("target", "_blank");//this makes your edit window open on new tab

            e.Row.Cells[0].Controls.Add(LB);
        }
        catch { }
    }

if(Request.QueryString["id"])!=null)
{
    string ID = Request.QueryString["id"];
}