problem with my Microsoft word 2007 com object .

Question

1) i have made a reference to microsoft.Office.Interop.Word
2) included a namespace has "include Word=microsoft.Office.Interop.Word" and given Word as alias to the above namespace
3) Word.Application WordApp = new Word.Application();
Word.Document WordDoc = null;
created an instance for Application and Document class
4) object missing =System.Reflection.Missing.Value
5)WordDoc = WordApp.Documents.Open(ref filename, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing);

here i given valid path for the file to be opened. but still my WordDoc gives me null vaule. (i have cross checked my filepath twice)

kindly help me where i'm going wrong.
thanks in advance.