You can't do this:
string config = File.ReadAllText(config + "/Settings.cfg");
Since that line declares the variable
, you can't use it's value in the assignment as it doesn't have any value yet!
That won't even compile, and for good reason ...
string path = @"C:\Path\";
string file = "app_path.Text";
string fullPath = Path.Combine(path, file);