Why can't set cast an object from Excel interop?

Question

Trying to get a reference to the worksheets (using Excel interop):

Excel.Application xl = new Excel.ApplicationClass();
Excel.Workbooks xlWorkBooks = xl.Workbooks;
Excel.Workbook xlWorkBook = xlWorkBooks.Open(fileName, 0, false, 5, "", 
                      "", true, Excel.XlPlatform.xlWindows, "\t",
                      false, false, 0, true, 1, 0);

// Next line crashes
Excel.Worksheets xlWorkSheets = (Excel.Worksheets) xlWorkBook.Worksheets; 

The error is that it cannot cast it:

Unable to cast COM object of type 'System.__ComObject' to interface type 'Microsoft.Office.Interop.Excel.Worksheets'. This operation failed because the QueryInterface call on the COM component for the interface with IID '{000208B1-0000-0000-C000-000000000046}' failed due to the following error: No such interface supported (Exception from HRESULT: 0x80004002 (E_NOINTERFACE)).

Is my cast incorrect?

Solution 1

Yes, your cast is wrong.

_Workbook.Sheets gives you a Sheets instance. This interface gives you all types of sheets, not just worksheets; mainly, it includes charts, macro sheets, etc.

On the other hand, the Worksheets interface only gives you worksheets - not charts.

The interfaces are not assignable to each other; therefore, you get the COM error. It's confusing - I'm not even sure if it's possible to get an instance of the Worksheets interface through the PIA - but that's Office Interop for ya.

As long as you use the _Workbook.Worksheets property instead of the _Workbook.Sheets property, you should get an instance of Sheets that only returns Worksheet objects - in spite of the fact that the interface is capable of providing other types of sheets.

Solution 2

According to MSDN, Workbook.Worksheets returns Microsoft.Office.Interop.Excel.Sheets.

So you'd cast it like this:

Microsoft.Office.Interop.Excel.Sheets sheets = 
    (Microsoft.Office.Interop.Excel.Sheets)xlWorkBook.Worksheets

Or assuming Excel maps to Microsoft.Office.Interop.Excel (as appears from your question)

Excel.Sheets sheets = (Excel.Sheets)xlWorkBook.Worksheets

Solution 3

If it works on one environment but not another, check the HK Classes Root/TypeLib reg keys.

It is possible that you're trying to run for HKCR\TypeLib{00020813-0000-0000-C000-000000000046}\1.6 but something the user installed has added the key: HKCR\TypeLib{00020813-0000-0000-C000-000000000046}\1.7 causing the Interop call to throw an exception.

Or if that isn't it, it could be something in the GAC because of different OS versions.

I had this issue where it worked on our developer machines running Windows 7, and caused this error on a user's machine running XP.

Solution 4

Odd one. According to this page, it's supposed to be of type Sheets not Worksheets. Haven't tested - give it a whirl?

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Comments

[Aaronaught]

This question is tagged .net-2.0, which means he can't use Linq to XML or any of the packaging classes. Even if he could, this still wouldn't answer the question.

[AngryHacker]

@Zach & @Joel are right too, but you were the first to the starting gate, the the green check.

[TheBlastOne]

Even though this is quite well-documented, it might be one of the questions/answers that make so worthwhile.

[Ross Brasseaux]

Thank you for this. The "gotcha" for me was that Excel.Workbook.Worksheets returns an Excel.Sheets object—not an Excel.Worksheets object.