wordApp.Documents.Open（）获不到对象 请大神求救

wordApp.Documents.Open（）获不到对象 请大神求救
  bool isSucceed = false;
            Word.WdExportFormat fileFormat = Microsoft.Office.Interop.Word.WdExportFormat.wdExportFormatPDF;
            Word._Application wordApp = null;
            if (wordApp == null)
            {
                wordApp = new Word.Application();
            }
           Word._Document wordDoc = null;

            try
            {             
                wordDoc = wordApp.Documents.Open(fileName);                wordDoc.ExportAsFixedFormat(outputFileName, fileFormat);
                isSucceed = true;
            }
为什么WordDoc的值为空，fileName的值是存在的，权限我也设置了。

代码上看，都是空值啊。 能设断点吗？ 

wordApp.Documents.Open(ref templateFile, ref Nothing, ref Nothing, ref Nothing, ref Nothing, ref Nothing, ref Nothing, ref Nothing, ref