public void ExportToExcel()
    {

        DataTable tblFiltered = _currentTable.AsEnumerable()
                          .Where(r => r.Field<string>("Type") == ExportGridGroup)
                          .CopyToDataTable();

        StringBuilder sb = new StringBuilder();
        IEnumerable<string> columnNames = tblFiltered.Columns.Cast<DataColumn>().
                                          Select(column => column.ColumnName);
        sb.AppendLine(string.Join(",", columnNames));

        foreach (DataRow row in tblFiltered.Rows)
        {
            IEnumerable<string> fields = row.ItemArray.Select(field => """ + field.ToString() + """);
            sb.AppendLine(string.Join(",", fields));
        }

        String result = sb.ToString();
        try
        {
            StreamWriter sw = new StreamWriter("export.csv");
            sw.WriteLine(result);
            sw.Close();
            Process.Start("export.csv");
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }
    }

[TestMethod()]
    public void ExportToExcelTest()
    {
        //Step 1: Creating a mock table with columns
        DataTable testtable = new DataTable();
        DataRow mydatarow;
        mydatarow = testtable.NewRow();
        //Step 2: Adding the row
        mydatarow["ColumnA"] = "12345";
        mydatarow["ColumnB"] = "test";

        testtable.Rows.Add(mydatarow);

        StringBuilder sb = new StringBuilder();
        IEnumerable<string> columnNames = testtable.Columns.Cast<DataColumn>().
                                          Select(column => column.ColumnName);
        sb.AppendLine(string.Join(",", columnNames));

        foreach (DataRow row in testtable.Rows)
        {
            IEnumerable<string> fields = row.ItemArray.Select(field => """ + field.ToString() + """);
            sb.AppendLine(string.Join(",", fields));
        }

        String result = sb.ToString();
        StreamWriter sw = new StreamWriter("export.csv");
        sw.WriteLine(result);
        sw.Close();
        Process.Start("export.csv");
        Assert.Fail();
    }