Poll which line of code app is stuck at?

You can surround your code with some try-catching in troublesome places, and then print the stacktrace to a file in the catch-clause:

try
{
    // Error prone code
}
catch (System.Exception e)
{
    System.IO.File.WriteAllText("YourErrorFile.txt", e.StackTrace); 
}

This will shoot out the stacktrace to an error log you can later have a look at if your program crashes. The stacktrace should contain information on which method caused the crash and on which line. Keep in mind that the example is totally generic; in general you should never catch System.Exception because it'll swallow all exceptions and you won't know which precise one caused the crash. Instead, look up which exceptions your code might throw, then catch those. :P

Edit: It's probably a good idea to include some added information in a little header, such as DateTime.Now, so that you know which execution (and version of program) each error belongs to.