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How to update the excel file cell data, when the file is created using a template in C#?

I am creating a new excel file using a template file, but I am unable to edit the contents in the new file created, please assist with the same.
Thanks in advance.
FileStream fileStream = new FileStream(filepath, FileMode.Open, FileAccess.Read);
using (ExcelPackage package = new ExcelPackage(fileStream))
{
package.Save();
}
string name = "filecreated.xlsx";
string fileType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
fileStream.Position = 0;
//return file
return File(fileStream, fileType, name);```

It Seems like you are opening the file in read only access mode
FileStream fileStream = new FileStream(filepath, FileMode.Open,
FileAccess.Read);
Will you please try like
FileInfo existingFile = new FileInfo(filepath);
using (ExcelPackage package = new ExcelPackage(existingFile))
{
//As we have not modified the file, its useless to call Save()
//This will override the opened file
package.Save();
}

Updating the excelsheet using EPPlus in c#

I have a excel file which contains sheet named Data. This sheet already contains few data. I need to open this file and add more data in it. I have tried searching about this but everyone is just creating new sheets in their workbook. I need to update the current sheet. Below is my code:
MemoryStream ms = new MemoryStream();
using (FileStream fs = File.OpenRead(#"Path\File.xlsx")
using (ExcelPackage excelPackage = new ExcelPackage(fs))
{
ExcelWorkbook excelWorkBook = excelPackage.Workbook;
ExcelWorksheet excelWorksheet = excelWorkBook.Worksheets.First();
excelWorksheet.Cells[1, 1].Value = "Test";
excelWorksheet.Cells[3, 2].Value = "Test2";
excelWorksheet.Cells[3, 3].Value = "Test3";
excelPackage.SaveAs(ms);
}
but it didnt update the sheet. I do not what I am doing wrong. Can anyone help me please. Thanks

You are doing this:
Create a MemoryStream object in memory to store binary data.
Open file and read it into the MemoryStream object.
Create an ExcelPackage object based on the data in the MemoryStream object.
Make changes to the spreadsheet.
Save changes back to the MemoryStream object.
That's why the spreadsheet file does not get updated.
Use FileInfo and open the file directly with ExcelPackage:
// using System.IO;
FileInfo file = new FileInfo(#"Path\File.xlsx");
using (ExcelPackage excelPackage = new ExcelPackage(file))
{
ExcelWorkbook excelWorkBook = excelPackage.Workbook;
ExcelWorksheet excelWorksheet = excelWorkBook.Worksheets.First();
excelWorksheet.Cells[1, 1].Value = "Test";
excelWorksheet.Cells[3, 2].Value = "Test2";
excelWorksheet.Cells[3, 3].Value = "Test3";
excelPackage.Save();
}

Try this
var file = new FileInfo(path);
if (file .Exists)
{
using (ExcelPackage excelPackage = new ExcelPackage(file))
{
ExcelWorksheet ws = excelPackage.Workbook.Worksheets.First();
ws.Cells[1,1].Value = "Test1";
excelPackage.Save();
}
}

Copy sheet from one workbook to another using NPOI in C#

I need to copy a sheet from one workbook to another. I am trying with the below code, but it is not working:
ISheet newSheet = wb.GetSheetAt(0).CopySheet("WeeklyReport");
string filePath = "billing_template2.xlsx";
XSSFWorkbook billingWorkbook;
using (var fs = new FileStream(filePath, FileMode.Open, FileAccess.Read))
{
billingWorkbook = new XSSFWorkbook(fs);
}
billingWorkbook.Add(newSheet);
where wb is the source workbook and billingWorkbook is my destination workbook.
Note: My destination workbook already had a sheet. I need to add the copied sheet after this one.

Save an Excel file into Local Directory using c#

I am creating an Excel using EPPlus and Datatable.Now as per my requirement i have to save this into my local system directory but i am not able to get how to achieve this ..
The path where i have to save is C://Reports//excel.xls
Here is my code
using (ExcelPackage pck = new ExcelPackage(newFile))
{
ExcelWorksheet ws = pck.Workbook.Worksheets.Add(reportName);
ws.Cells["A1"].LoadFromDataTable(DT, true);
pck.Save();
}
Please help me ..

FileInfo newFile = new FileInfo(#"C:\Reports\excel.xls");
if (newFile.Exists)
{
newFile.Delete(); // ensures we create a new workbook
newFile = new FileInfo(#"C:\Reports\excel.xls");
}
using (ExcelPackage pck = new ExcelPackage(newFile))
{
ExcelWorksheet ws = pck.Workbook.Worksheets.Add(reportName);
ws.Cells["A1"].LoadFromDataTable(DT, true);
pck.Save();
}

ExcelPackage: Office Open XML

I am new to this Open Office XML and I was wondering what file extension the XLPackage takes.
For example I assumed I just needed to input the file location of a CSV file I am using, but it does not work, do I have to convert the file to .xlsx or is there something other then the XLPackage that I should use?
The problem is that once it gets to the using a new OpenDialog is initiated and I cant find my file. I am probably just missing something obvious. File Contains Corrupt data, FileFormatException, I assume I need to convert the file before use?
I appreciate any feedback.
Some code:
FileInfo existingFile = new FileInfo(eFilePath);
using (ExcelPackage xlPackage = new ExcelPackage(existingFile)) // I think the issue is here.
{
ExcelWorksheet exeedSheet = xlPackage.Workbook.Worksheets[1];
//Total rows
for (int row = 1; row > 0; )

If you are working with a CSV the ExcelPackage is overkill for what you are doing.
CSV:
using (var Sr = new StreamReader("\\SomeCoolFile.CSV"))
{
var text = Sr.ReadToEnd();
Sr.Close();
text = text.Replace("\n", string.Empty);
var lines = text.Split('\r');
var info = lines.Select(line => line.Split(',')).ToList();
......
}
ExcelPackage:
using (var fs = new FileStream("\\SomeCoolFile.xlsx", FileMode.Open))
{
using (var package = new ExcelPackage(fs))
{
var workBook = package.Workbook;
.....
}
}