1. The cellnum in row.GetCell(cellnum) is actually fetched according to the order of the columns in Excel (starting from 0), not the data index in Cells.
Therefore, a temporarily filled List is usually required for matching. The filling achieves the meaning of the same index as Cells, and you can use this List to fetch according to the index.
2. Operate and save Excel during operation
book is equivalent to creating a stream in memory with the open file, modifying the stream, creating a file stream, and then writing the book into the file stream. The modification operation on Excel can be saved.