problem with bulk copying into sql server database

hello im trying to copy the contents of my datagridview into an sql server. here's the code

Code:

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void BulkInsertToSQL(DataTable dt, string tableName)
        {

                using (SqlConnection con = new SqlConnection(_DB))
                {
                    SqlBulkCopy sbc = new SqlBulkCopy(con);
                    sbc.DestinationTableName = tableName;

                    //if your DB col names don’t match your data table column names 100%
                    //then relate the source data table column names with the destination DB cols
                    sbc.ColumnMappings.Add("DBAttributeName1", "DTColumnName1");
                    sbc.ColumnMappings.Add("DBAttributeName2", "DTColumnName2");
                    sbc.ColumnMappings.Add("DBAttributeName3", "DTColumnName3");
                    sbc.ColumnMappings.Add("DBAttributeName4", "DTColumnName4");

                    con.Open();

                    sbc.WriteToServer(dt);
                    con.Close();
                }

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the problem is that my datagridview is populated from an external source which has variable number of rows.how can i do that column mapping thing here?
i would also like to know if there's any alternative to my approach