Crash: mutating method sent to immutable object

Question

I am attempting to delete an object for a particular key in an NSMutableDictionary but the application crashes when trying to do so. Here is my code:

    NSArray *allWebDictionary = [NSJSONSerialization JSONObjectWithData:webData options:0 error:nil];
    self.results = [[NSMutableArray alloc] initWithArray:allWebDictionary];

    for (NSMutableDictionary *dic2 in self.results) {
        NSMutableArray *incidents = [dic2 valueForKey:@"incidents"];
        for (NSMutableDictionary *incident in incidents) {
            [incident removeObjectForKey:@"type"];
        }
    }

My app crashes with the following log:

* Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '-[__NSCFDictionary removeObjectForKey:]: mutating method sent to immutable object'

Any ideas? Thanks in advance!

Have you tried specifying NSJSONReadingMutableContainers with options instead of 0? Always better to use the provided constants. Also, stick in a break point and check to see the real data type after you get the values. — DBD

Jeffery Thomas Jeffery Thomas · Accepted Answer · 2014-02-24T14:00:25

You made a mutable copy of the array, but the dictionary which is in the array is still immutable.

You need mutable containers.

Replace

NSArray *allWebDictionary = [NSJSONSerialization JSONObjectWithData:webData
                                                            options:0
                                                              error:nil];

with

NSMutableArray *allWebDictionary = [NSJSONSerialization JSONObjectWithData:webData
                                                                   options:NSJSONReadingMutableContainers
                                                                     error:nil];

Crash: mutating method sent to immutable object

1 Answers