Asp.net(c#)的xlApp.Workbooks.Open()问题,急!!

Excel.Application  xlApp  =  new  Excel.Application()  ;  
Excel.Workbook  xlBook  ;  
Excel.Worksheet  xlSheet  ;  
 
 
object  Missing=System.Reflection.Missing.Value;    
 
string  FileName  =    "excel/"  +  this.List_Class.SelectedValue  +  ".xls";  
if  (  File.Exists(Server.MapPath(FileName))  )  
{  
     File.Delete(Server.MapPath(FileName));  
}  
 
File.Copy(Server.MapPath("tmpl/St.xls"),  Server.MapPath(FileName));  
 
 
xlBook  =  xlApp.Workbooks.Open(Server.MapPath(FileName),Missing,Missing,Missing,Missing,Missing,Missing,Missing,Missing,Missing,Missing,Missing,Missing);  
                                     
xlSheet  =  (Excel.Worksheet)xlBook.Worksheets["Sheet1"];  
 
 
当执行到  xlApp.Workbooks.Open(                )  
出现异常:  
未将对象引用设置到对象的实例。  
 
请各位指点,不胜感激!