private string _filePath = Path.GetDirectoryName(System.AppDomain.CurrentDomain.BaseDirectory);

The method above will bring you something like this:

"C:\Users\myuser\Documents\Visual Studio 2015\Projects\myProjectNamespace\bin\Debug"

From here you can navigate backwards using System.IO.Directory.GetParent:

_filePath = Directory.GetParent(_filePath).FullName;

1 time will get you to \bin, 2 times will get you to \myProjectNamespace, so it would be like this:

_filePath = Directory.GetParent(Directory.GetParent(_filePath).FullName).FullName;

Well, now you have something like “C:\Users\myuser\Documents\Visual Studio 2015\Projects\myProjectNamespace”, so just attach the final path to your fileName, for example:

_filePath += @"\myfile.txt";
TextReader tr = new StreamReader(_filePath);

Hope it helps.