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Using c#.net can open excel workbook with console app but not with a windows forms app

I have been able to open an excel workbook using a console app in Visual Studio 2019 with the following code:
using Excel = Microsoft.Office.Interop.Excel;
private void OpenXL2()
{
Excel.Application excelApp = new Excel.Application();
if (excelApp == null)
{
return;
}
// open an existing workbook
string workbookPath = #"C:\Illustrator\InvoiceTemplates\invtemplate.xls";
Excel.Workbook excelWorkbook = excelApp.Workbooks.Open(workbookPath);
excelApp.Visible = true;
// get all sheets in workbook
Excel.Sheets excelSheets = excelWorkbook.Worksheets;
// get some sheet
string currentSheet = "TimeSheet";
Excel.Worksheet excelWorksheet =
(Excel.Worksheet)excelSheets.get_Item(currentSheet);
}
but when I try to use the same code in a windows form app the following error occurs when the excelApp.Workbooks.Open(workbookPath) is reached:
System.InvalidCastException: 'Unable to cast COM object of type 'Microsoft.Office.Interop.Excel.ApplicationClass' to interface type 'Microsoft.Office.Interop.Excel._Application'. This operation failed because the QueryInterface call on the COM component for the interface with IID '{000208D5-0000-0000-C000-000000000046}' failed due to the following error: Class not registered (Exception from HRESULT: 0x80040154 (REGDB_E_CLASSNOTREG)).'
I have added a reference to the "Microsoft Excel 16.0 Object Library"
Thanks.

try like that
Type excel_type = Type.GetTypeFromProgID("Excel.Application");
dynamic excel_obj = Activator.CreateInstance(excel_type);
var workbook = excel_obj.Workbooks.Open(Filename: filePath, ReadOnly: false);

C# Excel - How to switch to existing Worksheet and edit it?

So I open an existing Excel Application with a few Worksheets and add one Worksheet and edit it. That works perfectly fine.
Now I try to switch to another Worksheet to edit this one. Code here:
Excel.Worksheet OptimaPruefliste = OptimaWorkbook.Worksheets.get_Item(1);
OptimaPruefliste.Activate();
try
{
OptimaPruefliste.Range["A1:ZZ9999"].Borders.LineStyle = true;
OptimaPruefliste.Range["A1:ZZ9999"].Interior.Color = XlRgbColor.rgbWhite;
}
catch (Exception ex)
{
MessageBox.Show(ex.ToString());
}
It doesn´t throw an exception and it does get "OptimaPruefliste" as active sheet but it just doesn´t change anything.

So these three lines did the job:
OptimaPruefliste = (Worksheet)OptimaExcelApp.Worksheets.get_Item(1);
OptimaPruefliste.Activate();
OptimaPruefliste = OptimaExcelApp.ActiveSheet as Excel.Worksheet;

System.Runtime.InteropServices error when exporting dataGridView to Excel workbook

I have an application that displays certain database data, and includes a function to save that data to an excel workbook on request using the Microsoft.Office.Interop.Excel assembly. One of my users reports the following error when trying to save to an excel workbook:
System.Runtime.InteropServices.COMException (0x8002000B): Invalid index. (Exception from HRESULT: 0x8002000B (DISP_E_BADINDEX))
at Microsoft.Office.Interop.Excel.Sheets.get__Default(Object Index)
at WorkCalendar.Form1.saveBtn_Click(Object sender, EventArgs e)
We've verified that he does have Excel 2013 installed and all of the necessary assemblies came through ok according to the full exception details, so I hope one of you can shed some light on what's going on here.
Here's the saveBtn_Click event method mentioned in the error above (edited for conciseness)
private void saveBtn_Click(object sender, EventArgs e)
{
// creating Excel Application
_Application app = new Microsoft.Office.Interop.Excel.Application();
_Workbook workbook;
_Worksheet worksheet;
string fetchString = fetch.ToString("HH.mm.ss");
try
{
// Check for existing workbook and add new page
}
catch
{
// If no workbook found, create a brand new one
workbook = app.Workbooks.Add(Type.Missing);
worksheet = null;
worksheet = workbook.Sheets["Sheet1"];
}
try
{
// do not show the excel sheet being created
app.Visible = false;
worksheet = workbook.ActiveSheet;
worksheet.Name = fetchString;
// Get dataGridView data, insert it into the excel worksheet and format it
}
catch { }
finally
{
// save the application
// Exit from the application
app.Quit();
}
}
As I said, the application works fine on my and other computers on which it's been tested. Any ideas?
EDIT: Altered code example slightly to show the method looking for an existing workbook, and creating one if no workbook found.

Closing an open excel workbook in C#

There is one issue that I have been having, and I am trying to fix it because if the program crashes (the excel file stays open in the background), or the user has the excel workbook already open the program will crash because it is unable to open an already opened workbook.
Was trying to counter this issue by using the method from this question : C#: How can I open and close an Excel workbook?
But much to my dismay no success.
With this setup I get an error at wb.Close(true) saying I cannot use an unassigned local variable. To me it kind of makes sense, but I don't see how that is the case. It's not like an if statement where if the condition isn't met it doesn't jump in the loop. The try block will always execute.
Excel.Workbook wb;
try
{
wb = exApp.Workbooks.Open(#file);
}
catch (Exception)
{
wb.Close(true);
}
I also tried this way :
Excel.Workbook wb = new Excel.Workbook();
try
{
wb = exApp.Workbooks.Open(#file);
}
catch (Exception)
{
wb.Close(true);
}
but this time, I get a error: 80040154 Class not registered on the line Excel.Workbook wb = new Excel.Workbook(); when running the program. again... don't know why.
Any help is greatly appreciated.

Try this:
Excel.Workbook wb = null;
try
{
wb = exApp.Workbooks.Open(#file);
}
catch (Exception)
{
if (wb != null) wb.Close(true);
}

You want finally instead of catch. A finally block will always execute, whether there is an exception or not. Even if there isn't an Exception thrown, you still want to close the workbook to clear up the resources.
Something like this should be what you need.
Excel.Workbook wb = new Excel.Workbook();
try {
wb = exApp.Workbooks.Open(#file);
//More code...
}
catch (Exception ex) {
// Do any error handling you need/want to here.
}
finally {
// If there's a way to determine if the workbook is open, try that first.
wb.Close(true);
}

Microsoft.Office.Interop.Excel for Office 2007

I want to try the Windows form application that converts a office file (Excel, Word, Powerpoint) into a PDF file.
My client's PC will not install Visual Studio and Office version is 2007.
My application uses Microsoft.Office.Iterop.Excel.dll to covert to the PDF format.
This dll file cannot be found on my client's PC and an error has occurred as following.
System.AugumentException: Value does not fall within the expected range.
at Microsoft.Office.Interop.Excel._Workbook.ExportAsFixedFromat(.......)
How can I solve this problem?
My code is following
public bool ExportWorkbookToPdf(string workbookPath, string outputPath)
{
// If either required string is null or empty, stop and bail out
if (string.IsNullOrEmpty(workbookPath) || string.IsNullOrEmpty(outputPath))
{
return false;
}
// Create COM Objects
Microsoft.Office.Interop.Excel.Application excelApplication;
Microsoft.Office.Interop.Excel.Workbook excelWorkbook;
// Create new instance of Excel
excelApplication = new Microsoft.Office.Interop.Excel.Application();
// Make the process invisible to the user
excelApplication.ScreenUpdating = false;
// Make the process silent
excelApplication.DisplayAlerts = false;
// Open the workbook that you wish to export to PDF
excelWorkbook = excelApplication.Workbooks.Open(workbookPath);
MessageBox.Show(workbookPath);
// If the workbook failed to open, stop, clean up, and bail out
if (excelWorkbook == null)
{
excelApplication.Quit();
excelApplication = null;
excelWorkbook = null;
MessageBox.Show("in null");
return false;
}
var exportSuccessful = true;
try
{
// Call Excel's native export function (valid in Office 2007 and Office 2010, AFAIK)
excelWorkbook.ExportAsFixedFormat(Microsoft.Office.Interop.Excel.XlFixedFormatType.xlTypePDF, outputPath);
}
catch (System.Exception ex)
{
MessageBox.Show(ex.Message);
// Mark the export as failed for the return value...
exportSuccessful = false;
// Do something with any exceptions here, if you wish...
// MessageBox.Show...
}
finally
{
// Close the workbook, quit the Excel, and clean up regardless of the results...
excelWorkbook.Close();
excelApplication.Quit();
excelApplication = null;
excelWorkbook = null;
}
// You can use the following method to automatically open the PDF after export if you wish
// Make sure that the file actually exists first...
if (System.IO.File.Exists(outputPath))
{
MessageBox.Show(outputPath);
System.Diagnostics.Process.Start(outputPath);
}
return exportSuccessful;
}

As already said in the comments, each client needs to install the 2007 Microsoft Office Add-in: Microsoft Save as PDF. It doesn't matter if you are on Windows 8.1 or have any PDF reader installed. You need that Add-in to write PDFs with Office 2007. (You don't need any Add-In with Office 2010 or 2013.)