通过调用“评估”检查工作表是否存在，此技巧是否存在任何缺陷？

Question

I need to check whether a sheet with some specific name exists in a given workbook.

The naive way to do this is something like:

using Excel = Microsoft.Office.Interop.Excel;

bool ContainsSheet (Excel.Workbook workbook, string sheetName)
{
    try
    {
        Excel.Worksheet sheet = workbook.get_Item(sheetName)
                                as Excel.Worksheet;
        return sheet != null;
    }
    catch (System.Runtime.InteropServices.COMException ex)
    {
        return false;
    }
}

But that exception is annoying. It is wasting my time once and again while debugging other unrelated parts of my program.

I also want to avoid iterating through every worksheet of the workbook, comparing names. That looks to me profoundly inefficient.

After some research, I made up this solution, whis is based on the fact that Evaluate() returns an error code when it fails instead of throwing an exception:

using Excel = Microsoft.Office.Interop.Excel;

bool ContainsSheet (Excel.Workbook workbook, string sheetName)
{
    // Sadly, I need a sheet to call Evaluate

    Excel.Worksheet someSheet = workbook.Worksheets[1]
                                as Excel.Worksheet;

    if (someSheet == null)   // Is this even possible?
        return false;

    // Try to get a range referring the first cell (upper-left corner). Note that
    // Evaluate() returns a number if an error occurs...

    Excel.Range someRange = someSheet.Evaluate("\'"+sheetName+"\'!A1")
                            as Excel.Range;

    return someRange != null;
}

But this fails if the "R1C1 reference style" is activated in Excel (menu: Tools / Options / General). Taking that into account...

using Excel = Microsoft.Office.Interop.Excel;

bool ContainsSheet (Excel.Workbook workbook, string sheetName)
{
    // Sadly, I need a sheet to call Evaluate

    Excel.Worksheet someSheet = workbook.Worksheets[1]
                                as Excel.Worksheet;

    if (someSheet == null)   // Is this even possible?
        return false;

    // Try to get a range referring the first cell (upper-left corner). Note that
    // Evaluate() returns a number if an error occurs...

    Excel.Range someRange = someSheet.Evaluate("\'"+sheetName+"\'!A1")
                            as Excel.Range;

    if (someRange != null)
        return true;

    // Try again with the alternative "R1C1 reference style", which can be activated
    // in the menu: Tools / Options / General

    someRange = someSheet.Evaluate("\'"+sheetName+"\'!R1C1")
                as Excel.Range;

    return someRange != null;
}

I know I could check ReferenceStyle first and then call Evaluate() just once with the correct style. Something like:

Excel.Application excel = ExcelDna.Integration.ExcelDnaUtil.Application as Excel.Application;
System.Nullable<Excel.XlReferenceStyle> style = excel.ReferenceStyle as System.Nullable<Excel.XlReferenceStyle>;
string corner = style == null                          ? null :
                style == Excel.XlReferenceStyle.xlA1   ? "A1" :
                style == Excel.XlReferenceStyle.xlR1C1 ? "R1C1" : null;

Anyway, my question is: Is there any other flaw in my ContainsSheet() function?

UPDATE : The method proposed here takes a very short time (about 30 us) when the sheet exists, but a long time when it does not exist (about 150 us). Evaluate() must be provoking and catching an exception internally. Instead, iterating through the Sheets collection, as proposed by DGibbs below, takes even shorter times when there are just a few sheets (13 us, no matter whether the sheet exists or not). But these timings grow with the number of sheets. With 77 sheets, if the sheet searched is one of the last ones or does not exist, iterating takes about 200 us. However, that's a lot of sheets!

Answer 1

Could you not do something like:

public static bool ContainsSheet(this Excel.Workbook workbook, string sheetName)
{
     if(workbook.Sheets == null || !workbook.Sheets.Any())
           return false;

     foreach (var sheet in workbook.Sheets)
     {
          if (sheet.Name.Equals(sheetName))
          {
               return true;
          }
     }

     return false;
}

Much simpler and is a handy extension method.

var hasSheet = workbook.ContainsSheet("foo");