Edit existing excel files ASP.Net without losing formatting - asp.net

Question

I have a situation where I need to use an Excel Template and insert values, then allow the user to download the updated file. I have done this using ClosedXML but as it uses a non native format for xlsx, I get a format error when opening the downloaded document and the saved instance of the document looses all formatting from the template.
I need a solution that outputs a xlsx document that is a copy of the original template, formatting and all, with the additional inserted values. If necessary, I can make a temporary copy of the template on the server.
Is there a tool I can use for this purpose?

Answer 1

I ended up using OpenXML. I am using VB.Net for this project and as I found very little example code in VB (mostly java or c#), I will attach it so it can be used as a reference.
The really nice thing about it is the document you get out of it does not have any validation errors or anything and all sheet formatting (text justification, font face, etc) survives.
Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
Dim path = Server.MapPath("~/ExcelTemplates/rfi.xlsx")
Dim templateBytes As Byte() = System.IO.File.ReadAllBytes(path)
Using templateStream As MemoryStream = New MemoryStream
templateStream.Write(templateBytes, 0, templateBytes.Length)
Using sheetInstance As SpreadsheetDocument = SpreadsheetDocument.Open(templateStream, True)
Dim worksheetPart As WorksheetPart = GetWorksheetPartByName(sheetInstance, "Sheet1")
If worksheetPart IsNot Nothing Then
'this writes "Test!!!" to cell C7
'Dim cell As Cell = GetCell(worksheetPart.Worksheet, "C", 7)
'cell.CellValue = New CellValue("Test!!!")
'another way of doing it
GetCell(worksheetPart.Worksheet, "C", 7).CellValue = New CellValue("Test12")
'this next line would change the cell's contents data type
'cell.DataType = New EnumValue(Of CellValues)(CellValues.String)
worksheetPart.Worksheet.Save()
templateStream.Position = 0
Using ms As MemoryStream = New MemoryStream
templateStream.CopyTo(ms)
Response.Buffer = True
Response.Clear()
Response.ContentType = "application/vnd.ms-excel"
Response.AppendHeader("Content-Disposition", "filename=RFI.xlsx")
ms.WriteTo(Response.OutputStream)
Response.End()
End Using
End If
End Using
End Using
End Sub
Private Shared Function GetWorksheetPartByName(ByVal document As SpreadsheetDocument, ByVal sheetName As String) As WorksheetPart
Dim sheets As IEnumerable(Of Sheet) = document.WorkbookPart.Workbook.GetFirstChild(Of Sheets)().Elements(Of Sheet)().Where(Function(s) s.Name = sheetName)
If sheets.Count() = 0 Then
Return Nothing
End If
Dim relationshipId As String = sheets.First().Id.Value
Dim worksheetPart As WorksheetPart = CType(document.WorkbookPart.GetPartById(relationshipId), WorksheetPart)
Return worksheetPart
End Function
Private Shared Function GetCell(ByVal worksheet As Worksheet, ByVal columnName As String, ByVal rowIndex As UInteger) As Cell
Dim row As Row = GetRow(worksheet, rowIndex)
If row Is Nothing Then Return Nothing
Return row.Elements(Of Cell)().Where(Function(c) String.Compare(c.CellReference.Value, columnName & rowIndex, True) = 0).First()
End Function
Private Shared Function GetRow(ByVal worksheet As Worksheet, ByVal rowIndex As UInteger) As Row
Return worksheet.GetFirstChild(Of SheetData)().Elements(Of Row)().Where(Function(r) CType(r.RowIndex, UInteger) = rowIndex).First()
End Function