Text File Reading problem

 i am reading a text file
string filename="format01.txt";
    FileStream aFile = new FileStream( filename, FileMode.Open); this is working fine
 but i pass  file name from other page
string filename=Request.QueryString["ParamName"];
    FileStream aFile = new FileStream( filename, FileMode.Open);
the error message is
An object reference is required to access non-static member
how to get file name in and how to use get: set method
string filename=Request.QueryString["ParamName"];
    FileStream aFile = new FileStream( filename, FileMode.Open);
 
 
...