title: Class not registered error when creating Excel workbook in C#
body: <p>When I try to access an Excel spreadsheet using the following code I get a "Library not registered' error when defining the workbook object wrkbuk using C# from Visual Studio 2012 with Office 2007 (ver 12) installed</p>
  
  <pre><code>Microsoft.Office.Interop.Excel.Application excapp = new Microsoft.Office.Interop.Excel.Application();
          string bookname = @"C:\Users\Public\Documents\RECRUITMENT &amp; SELECTION\MOVEMENTS\MOVEMENTS\Miscellaneous Documents\VacanciesREAL.xls";
          Workbook wrkbuk = excapp.Workbooks.Open(bookname); 
          Worksheet wrksht = new Worksheet();
  </code></pre>
  
  <p>The error details are</p>
  
  <blockquote>
    <p>System.InvalidCastException was unhandled   HResult=-2147467262<br>
    Message=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: Library not registered. (Exception from HRESULT:
    0x8002801D (TYPE_E_LIBNOTREGISTERED)).   Source=mscorlib</p>
  </blockquote>
  
  <p>I've created a reference to Microsoft.Office.Interop.Excel.dll in  the Office 14 Primary Interop Assembly that comes with VS 2012 and also for the Office 12 version of the dll and neither fixes the problem. I've tried registering the dlls with regasm but this does not help either.</p>
  
  <p>I can create the worksheet wrksht on the following line OK with the Office 14 and Office 12 dlls so the problem seems only affect workbook definition.</p>
  
  <p><strong>Dec 17 2013</strong></p>
  
  <p>Tried reinstalling Office 2007 to no avail but found this solution, which worked. It was at</p>
  
  <p><a href="http://social.msdn.microsoft.com/Forums/vstudio/en-US/d3f92da7-96d3-404b-89d0-d236800ceae5/vs-2012-rc-and-visual-studio-tools-for-office?forum=vsto" rel="noreferrer">http://social.msdn.microsoft.com/Forums/vstudio/en-US/d3f92da7-96d3-404b-89d0-d236800ceae5/vs-2012-rc-and-visual-studio-tools-for-office?forum=vsto</a></p>
  
  <p>Problem is code from multiple versions of Office - I had code from Office 14, possibly coming from VS 2012 install</p>
  
  <p>I suspect that VS 2012 RC has installed the Office 2013 type libraries, and that there are now duplicate versions registered. I got that specific error message </p>
  
  <blockquote>
    <p>('unable to cast
    _Application'....'TYPE_E_LIBNOTREGISTERED')</p>
  </blockquote>
  
  <p>after a messy uninstall, and then re-installing an older version of Office. I solved it by following the advice of another forum thread here which I'm sadly unable to find...basically I searched the registry for the CLSID in the error message, so, <code>{00020970-0000-0000-C000-000000000046}</code>. Its registry key contained two keys, one of which was called 'TypeLib', which in turn contained another CLSID for the type library. I then searched the registry, starting at the beginning again, for that second CLSID, which led me to the relevant interop type library. It had two entries....8.3 and 8.4, the bigger number corresponding to the later version of Office....which I deleted...and immediately was able to run my program.</p>