Swift file reading permission error on macOS sandb

I'm trying to read the contents of a file on the filesystem in a macOS Swift app (Xcode 9 / Swift 4).

I'm using the following snippet for it:

let path = "/my/path/string.txt"
let s = try! String(contentsOfFile: path)
print(s)

My problem is the following:

This works in a Playground
This works when I use the Command Line Tool macOS app template
This terminates in a permission error when I use the Cocoa App macOS app template

The permission error is the following:

Fatal error: 'try!' expression unexpectedly raised an error:

Error Domain=NSCocoaErrorDomain Code=257 "The file "data.txt" couldn't be opened because you don't have permission to view it."

UserInfo={NSFilePath=/my/path/data.txt, NSUnderlyingError=0x60c0000449b0 {Error Domain=NSPOSIXErrorDomain Code=1 "Operation not permitted"}}

I guess it's related to sandboxing but I found no information about it.

How can I read from the filesystem in a sandboxed app? I mean there are so many GUI apps which need an Open File dialog, it cannot be a realistic restriction of sandboxed apps to not read files from outside the sandbox.
Alternatively, how can I switch off sandboxing in Build Settings?
Finally, I tried to compare the project.pbxproj files between the default Cocoa Apps and Command Line Tool template and I didn't see any meaningful difference, like something about security or sandbox. If not here, where are those settings stored?

Your bullet 1: you have the answer, you need to use NSOpenPanel to obtain the user's permission to access the file. You can use NSOpenPanel methods to restrict the user's choice to the cancel button and just the file you wish - thus making a "request access to" dialog. Start with Apple's App Sandbox Design Guide.

Your bullet 2: you've the right idea, just the wrong tab, you should find the sandboxing switch on the Summary tab.

HTH