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Asked: 2026-05-27T14:53:07+00:00

New to VSTO, know Excel’s model very well… I have using Excel = Microsoft.Office.Interop.Excel;

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New to VSTO, know Excel’s model very well… I have 

using Excel = Microsoft.Office.Interop.Excel;

at the head of my class, and the following simple call, which compiles correctly and even appears correct in intellisense

double usedRows = (double)Excel.WorksheetFunction.CountA(oWks.Columns[1]);

but fails at runtime with this exception

‘Microsoft.Office.Interop.Excel.WorksheetFunction’ does not contain a
definition for ‘CountA’

about which I can find little further information. What am I missing?

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1 Answer

  1. Editorial Team

    CountA needs to reference your Excel application instance, not the namespace (I think that’s the correct terminology).

    So assuming you created or got and instance of Excel and called it oExcel then it’s:

    double usedRows = (double)oExcel.WorksheetFunction.CountA(oWks.Columns[1])

    … although I’m not a C person, so can’t confirm that the “(double)” part is correct.

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