How to write a utility to work with a running window application?

Question

C++ / C#

I need to write a program - a utility, how can I implement data output to the window of another application, text data, I need to do something similar to choosing an open window in Windows and possibly choosing a part of the application window into which data will be displayed, all this in C #.
PS Fraps does something similar with showing FPS in the captured window.

Answer 1

On hooks, you roll up to the desired process, inject your code, and then, as a fantasy, you can draw with your code.
https://easyhook.github.io/