Hello amir1812,
Thank you for being part of the Apple Community!
It is my understanding you are experiencing an issue with your computer freezing when the file dialog box is opened. I have a couple of suggestions that hopefully will help you out.
The first thing I would do is backup everything that is on the computer. Nothing I am going to suggest should cause any data loss, but it is always a good idea to have a backup. Your computer has software on it called Time Machine. It is fantastic software for backing up your computer. Information on how to use Time Machine is available here:
Use Time Machine to back up or restore your Mac.
Once you have a backup the next step would be to test in Safe Mode.
Follow these steps to start up into safe mode.
- Start or restart your Mac.
- Immediately after you hear the startup sound, press and hold the Shift key.
- Release the Shift key when you see the Apple logo appear on the screen.
After the Apple logo appears, it might take longer than usual to reach the login screen or your desktop. This is because your Mac performs a directory check of your startup disk as part of safe mode.
If an issue doesn't happen in safe mode
If an issue doesn't happen when your Mac is started in safe mode, try restarting again without pressing any keys at startup. If the issue appears to be resolved when you start up normally, it was possibly caused by a cache or a directory issue with your startup disk that safe mode fixed.
If you restart your Mac normally and an issue comes back when you reach your desktop, try disabling any login items that automatically open when you log in.
More information on Safe Mode can be found here:
Try safe mode if your Mac doesn‘t finish starting up
If the issue continues, then we need to try to isolate what is causing the issue. Start by testing in a test user account. Follow the instructions found here:
How to test an issue in another user account on your Mac
If the issue no longer occurs with the test user account, that indicates that something that is starting up during the normal user account login is causing the problem. To solve this, remove items from the Login Items.
OS X El Capitan: If you think you have incompatible login items
Have a great day!