Question

NSDictionary *dict = [NSDictionary dictionaryWithObjectsAndKeys:@"Please, other format selected.", NSLocalizedDescriptionKey, @"This format is not available.", NSLocalizedFailureReasonErrorKey, @"The operation couldn't be completed.", NSUnderlyingErrorKey, self.URL, NSURLErrorKey, nil];

NSLog(@"dict:%@", dict); // no Crash

NSError *error = [NSError errorWithDomain:@"testError" code:1 userInfo:dict];// crash

why crashing when allocate error object?

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Here are the crash details:

+[__NSCFConstantString objectForKey:]: unrecognized selector sent to class 0x3ebf5550

Answer 1

IIRC NSUnderlyingErrorKey is supposed to be another instance of NSError.

From the docs:

The user info dictionary can sometimes include another NSError object
that represents an error in a subsystem underlying the error
represented by the containing NSError. You can query this underlying
error object to obtain more specific information about the cause of
the error.

You access the underlying error object by using the NSUnderlyingErrorKey dictionary key.