Inserting Row to GridView Error: Collection was modified enumeration operation might not execute

I am inserting values from textbox to gridview

but having error Collection was modified enumeration operation might not execute

C# 

        protected void add_Click(object sender, EventArgs e)
        {

            if(booktitle.Text == "")
            {
                Response.Write("<script language='javascript'>alert('Please Input Book Name');</script>");
            }
            else if (isbn.Text == "")
            {
                Response.Write("<script language='javascript'>alert('Please Input IBAN No');</script>");
            }
            else if(qty.Text == "")
            {
                Response.Write("<script language='javascript'>alert('Please Input QTY');</script>");
            }

            //add the selected item to gridview

            DataTable dtgridview = ViewState["dt"] as DataTable;
            DataRow dr2 = dtgridview.NewRow();
          
            foreach (DataRow dr in dtgridview.Rows)            
            {
                dr2["Name"] = booktitle.Text.Trim();
                dr2["IBAN"] = isbn.Text.Trim();
                dr2["QTY"] = qty.Text;
                dtgridview.Rows.Add(dr2);
                   
                ViewState["dt"] = dtgridview;
                GridView1.DataSource = dtgridview;
                GridView1.DataBind();                
            }
        }