How to access an already opened Excel file in C#?

Question

I have an excel workbook opened via double-clicking it in windows explorer but cannot access it in code

Excel.Application xlApp = (Application)Marshal.GetAct         


        

Answer 1

If all your workbooks are opened in the same Excel instance (you can check this by checking if you can switch from one to the other using Alt-tab). You can simply refer to the other using Workbooks("[FileName]"). So, for example :

Dim wb as Workbook //for C#, type Excel.Workbook wb = null;
Set wb = Workbooks("MyDuperWorkbook.xlsx") //for C#, type wb = Excel.Workbooks["MyDuperWorkbook.xlsx"];
wb.Sheets(1).Cells(1,1).Value = "Wahou!"

Answer 2

I know this thread is a little old, but I found another way of doing this. When you create a new Excel.Applicationobject you have to create the WorkBooks object. When you access an already opened Excel file, the WorkBooks object is already created, so you just need to add a new WorkBook to the existing one. @Tipx 's solution works great if you have access to the WorkBook name, but in my case the current WorkBook name is always random. Here's the solution I came up with to get around this:

Excel.Application excelApp = null;
Excel.Workbooks wkbks = null;
Excel.Workbook wkbk = null;

bool wasFoundRunning = false;

Excel.Application tApp = null;
//Checks to see if excel is opened
try
{
    tApp = (Excel.Application)System.Runtime.InteropServices.Marshal.GetActiveObject("Excel.Application");
    wasFoundRunning = true;
}
catch (Exception)//Excel not open
{
    wasFoundRunning = false;
}
finally
{
    if (true == wasFoundRunning)
    {
        excelApp = tApp;
        wkbk = excelApp.Workbooks.Add(Type.Missing);                    
    }
    else
    {
        excelApp = new Excel.Application();
        wkbks = excelApp.Workbooks;
        wkbk = wkbks.Add(Type.Missing);
    }
    //Release the temp if in use
    if (null != tApp) { Marshal.FinalReleaseComObject(tApp); }
    tApp = null;
}
//Initialize the sheets in the new workbook

Might not be the best solution but it worked for my needs. Hope this helps someone. :)