NSDictionary writeToFile fails while objects are valid, permission is 0k - objective-c

Question

Why NSDictionary cannot be written?? I have checked the content of the dictionary: all the instances are of NSString and NSNumber. I checked permissions: a text file with the same name at the same path is written well. Of course, my dictionary is not empty.
NSString *file = ...
NSDictionary *dict = ...
// check dictionary keys
BOOL wrong = NO;
for (id num in [dict allKeys]) {
if (![num isKindOfClass:[NSNumber class]]) {
wrong = YES;
break;
}
}
if (wrong) {
NSLog(#"First");
}
// check dictionary values
wrong = NO;
for (id num in [dict allValues]) {
if (![num isKindOfClass:[NSString class]]) {
wrong = YES;
break;
}
}
if (wrong) {
NSLog(#"Second");
}
if (![dict writeToFile:file atomically:YES]) {
// 0k, let's try to create a text file
NSLog(#"Names writing error!");
[#"Something here... .. ." writeToFile:file atomically:YES encoding:NSUTF8StringEncoding error:nil];
}
Output: "Names writing error!"
Text file is created successfully.

Answer 1

Writing out a dictionary creates a property list, and according to the documentation all keys in a property list must be strings.
... and although NSDictionary and CFDictionary objects allow their keys to
be objects of any type, if the keys are not string objects, the
collections are not property-list objects.
NSNumber objects as keys are not supported.

Answer 2

As #vadian points out, you cannot write plist with numeric keys. But you can use NSKeyedArchiver:
NSURL *documents = [[NSFileManager defaultManager] URLForDirectory:NSDocumentDirectory inDomain:NSUserDomainMask appropriateForURL:nil create:false error:nil];
NSURL *fileURL = [documents URLByAppendingPathComponent:#"test.plist"];
// this will not work
NSDictionary *dictionary = #{#1: #"foo", #2: #"bar"};
BOOL success = [dictionary writeToFile:fileURL.path atomically:true];
NSLog(#"plist %#", success ? #"success" : #"failure");
// this will
fileURL = [documents URLByAppendingPathComponent:#"test.bplist"];
success = [NSKeyedArchiver archiveRootObject:dictionary toFile:fileURL.path];
NSLog(#"archive %#", success ? #"success" : #"failure");
And you can read it back with NSKeyedUnarchiver:
// to read it back
NSDictionary *dictionary2 = [NSKeyedUnarchiver unarchiveObjectWithFile:fileURL.path];
NSLog(#"dictionary2 = %#", dictionary2);
Note, you can do this with any class that conforms (and properly implements) NSCoding. Fortunately, NSDictionary conforms already. You have to make sure that any objects inside the dictionary, also conform (both NSString and NSNumber do). If you had a custom object in your dictionary, you'd have to make it properly conform yourself.
This is all described in the Archives and Serializations Programming Guide.