Export DataGridView to Excel. You must have "export.xlt" file in project directory.

gistfile1.cs

public void ExportToExcel(DataGridView grid) {
  ApplicationClass Excel = new ApplicationClass();
  XlReferenceStyle RefStyle = Excel.ReferenceStyle;
  Excel.Visible = true;
  Workbook wb = null;
  String TemplatePath = System.Windows.Forms.Application.StartupPath + @"\export.xlt";
  try {
    wb = Excel.Workbooks.Add(TemplatePath); // !!! 
  } catch (System.Exception ex) {
    throw new Exception("Template is unavailable " + TemplatePath + "\n" + ex.Message);
  }
  Worksheet ws = wb.Worksheets.get_Item(1) as Worksheet;
  for (int j = 0; j < grid.Columns.Count; ++j) {
    (ws.Cells[1, j + 1] as Range).Value2 = grid.Columns[j].HeaderText;
    for (int i = 0; i < grid.Rows.Count; ++i) {
      object Val = grid.Rows[i].Cells[j].Value;
      if (Val != null) {
        (ws.Cells[i + 2, j + 1] as Range).Value2 = Val.ToString();
      }
    }
  }
  ws.Columns.EntireColumn.AutoFit();
  Excel.ReferenceStyle = RefStyle;
  ReleaseExcel(Excel as Object);
}

private void ReleaseExcel(object excel) {
  Marshal.ReleaseComObject(excel);
  GC.GetTotalMemory(true);
}