public void Validate_Month(DataGridView dataGridView, int month, int select)
    {
        decimal num;

        if (dataGridView.ColumnCount > 3)
        {
            for (int h = select; h <= month; h++)
            {
                for (int i = 0; i < dataGridView.RowCount; i++)
                {
                    if (!Decimal.TryParse(dataGridView[h, i].Value.ToString(), out num))
                    {
                        if (dataGridView[h, i].Value.ToString() == null || dataGridView[h, i].Value.ToString() == "")
                        {

                        }
                        else
                        {
                            MessageBox.Show("Row not decimal:" + " [ " + dataGridView[h, i].Value.ToString() + "] in column " + dataGridView.Columns[h].Name);
                        }
                    }
                }
            }
        }
    }
}

public void Validate_Month(DataGridView dataGridView, int month, int select)
    {
        decimal num;
        StringBuilder sb =new StringBuilder();
        if (dataGridView.ColumnCount > 3)
        {
            for (int h = select; h <= month; h++)
            {
                for (int i = 0; i < dataGridView.RowCount; i++)
                {
                    if (!Decimal.TryParse(dataGridView[h, i].Value.ToString(), out num))
                    {
                        if (dataGridView[h, i].Value.ToString() == null || dataGridView[h, i].Value.ToString() == "")
                        {

                        }
                        else
                        {
                            sb.AppendLine("Row not decimal:" + " [ " + dataGridView[h, i].Value.ToString() + "] in column " + dataGridView.Columns[h].Name);
                        }
                    }
                }
            }
        }

        MessageBox.Show(sb.ToString());
    }