C# Move File to folder -

i'm getting error "extensions" folder, not file, when trying run this

openfiledialog openfiledialog = new openfiledialog();         openfiledialog.filter = "msl files (.msl)|*.msl";         var onlyfilename = system.io.path.getfilename(openfiledialog.filename);         bool flag = openfiledialog.showdialog() == dialogresult.ok;         if (flag)         {             this.txtpath.text = openfiledialog.filename;             file.copy(txtpath.text, application.startuppath + @"\extensions\" + @onlyfilename);         }         extensiontree.refresh(); 

how fix?

you set onlyfilename before show dialog, means null (empty) doesn't added on path.