"Mutating method sent to immutable object" despite object being NSMutableDictionary

Question

I'm using NSMutableDictionary and hit this error:

'NSInternalInconsistencyException', reason: '-[__NSCFDictionary removeObjectForKey:]: mutating method sent to immutable object'

Here's the code:

    // Turn the JSON strings/data into objects
    NSError *error;
    NSMutableDictionary *invoiceDictFromReq = [[NSMutableDictionary alloc] init];
//    invoiceDictFromReq = (NSMutableDictionary *)[NSJSONSerialization JSONObjectWithData:[request responseData] options:kNilOptions error:&error];
    invoiceDictFromReq = [NSMutableDictionary dictionaryWithDictionary:[NSJSONSerialization JSONObjectWithData:[request responseData] options:kNilOptions error:&error]];

NSLog(@"invoiceDictFromReq count: %i, key: %@, value: %@", [invoiceDictFromReq count], [invoiceDictFromReq allKeys], [invoiceDictFromReq allValues]);

// Get values and keys from JSON response
self.invoiceDict = [invoiceDictFromReq objectForKey:@"invoice"];
NSNumber *invoiceAmount = [self.invoiceDict objectForKey:@"amount"];
NSNumber *invoiceId = [self.invoiceDict objectForKey:@"id"];
NSNumber *invoiceNumber = [self.invoiceDict objectForKey:@"number"];
NSNumber *checkoutStarted = [self.invoiceDict objectForKey:@"checkoutStarted"];
NSNumber *checkoutCompleted = [self.invoiceDict objectForKey:@"checkoutCompleted"];
NSLog(@"amount: %@, id: %@, number: %@, started: %@, completed: %@", invoiceAmount, invoiceId, invoiceNumber, checkoutStarted, checkoutCompleted);

All the console logs indicate that the data is fine. This is where things start to break down. I pass the invoiceDict property to the next view controller:

// Pass the invoice to checkoutViewController
[checkoutViewController setInvoiceDict:self.invoiceDict];

In CheckoutViewController.m:

    // Change invoice checkoutCompleted to true
//    [self.invoiceDict removeObjectForKey:@"checkoutCompleted"];
    [self.invoiceDict setObject:[NSNumber numberWithBool:YES] forKey:@"checkoutCompleted"];

The error is at [self.invoiceDict setObject...]. I made sure that all the dictionaries I use are NSMutableDictionary. I left some of the commented-out lines in the code to show the things I've tried and I hit a brick wall. I suppose I can always create a new dictionary. Is that the preferred way to do it?

Answer 1

NSJSONSerialization returns immutable objects by default. Here is how to get mutable dictionary from the parser:

• use option NSJSONReadingMutableContainers

or

• use mutableCopy on the result