excel关闭进程问题？？？

Question

我的代码：
Excel.Application app=new Excel.Application();
string filename=filePath;
object missing = System.Reflection.Missing.Value;
Excel._Workbook wb=app.Workbooks.Open(filename,missing,missing,missing,missing,missing,missing,missing,missing,missing,missing,missing,false);
Sheets sheets = app.Worksheets;
Excel._Worksheet worksheet = (Excel._Worksheet) sheets.get_Item(2);
// Excel._Worksheet worksheet = (Excel._Worksheet)wb.Worksheets["11"];
string aaa=worksheet.Name.ToString();
     
        
worksheet.Cells[3,3]="";
wb.Close(false,missing,missing);   
app.Quit( );
app=null;
missing=null;
sheets=null;
range2=null;
worksheet=null;
System.GC.Collect();
System.GC.WaitForPendingFinalizers();
}
不能关闭进程去掉worksheet.Cells[3,3]="";这句就关上了，加上关不了，大哥们给看看？？？？