You can do it like this:
DataView myview = new DataView(myset.test);
myview.RowFilter =string.Format( "productname ='{0}'" , Request.QueryString["productname"].ToString() );
DataTable dt = myview.ToTable();
if(dt.Rows.Count > 0){
TextBox1.Text = dt.Rows[0]["ColumnName"].ToString();
}