Streamreader to a relative filepath

Is the Folder "TestDir" always in the executable directory? if so, try this

    string dir =System.IO.Path.GetDirectoryName(
      System.Reflection.Assembly.GetExecutingAssembly().Location);

    string file = dir + @"\TestDir\TestFile.txt";

This will give you the path of the exe plus the folder inside it and the textfile

You can use the GetFullPath() method. Try this:

 string filePath = System.IO.Path.GetFullPath("TestFile.txt");
 StreamReader sr = new StreamReader(filePath);