using c# to select a worksheet in excel

You can use the following code :

Worksheet sheet = (Worksheet)xlApp.Worksheets[1];
sheet.Select(Type.Missing);

or

sheet.Activate();

I have used this code and it works fine for me.

Are your properties initialized?

If they are, you should probably be able to achieve what you are trying to by either of those:

xlApp.ActiveWorkbook.Sheets[1].Activate();
xlWorkbook.Sheets[1].Activate();
xlSheet.Activate();

If they are not, you should initialize at least xlApp property to Application object you're working with and then use the code above. You can initialize first two objects by using the code below.

xlApp = new Microsoft.Office.Interop.Excel.Application();
Workbooks xlWorkbooks = xlApp.Workbooks;
xlWorkbook = xlWorkbooks.Open(@"C:\filename.xlsx");

Here is what I did and it works!

Excel.Worksheet xlWorkSheetFocus = (Excel.Worksheet)xlWorkBook.Worksheets.get_Item(2);
xlWorkSheetFocus.Activate();