I am trying to model a wall. There will be some curves in the model, so I will need to apply Sub Surf modifier while keeping the corner edges sharp. Please find my model here: https://www.mediafire.com/file/tz4cmab53uleb16/1.blend/file
This is what my work looks like before the sub surf modifier:
This what it looks like even when I have marked the sharp edges:
I had expected it to keep the right angles and prevent them from bending. Was I wrong? I have also enabled Auto Smooth.
Please note that I have already seen this thread: