AFNetworking JSON Issue - objective-c

Question

I keep receiving the following error message: 2013-01-22 01:44:43.091 Section3App2[16625:6703] -[__NSCFArray length]: unrecognized selector sent to instance 0x23a48780 after submitting my AFNetworking Request. The idea behind the request is that we are sending a post request to a REST API with a JSON Request Body via POST. I have been fiddling with this all day and can't seem to figure out whats causing the problem.
CODE
NSString *string = #"[{\"code\": \"105N14560\"}]";
NSString * jsonString = string;
NSData * data = [jsonString dataUsingEncoding:NSUTF8StringEncoding];
NSError * error = nil;
id json = [NSJSONSerialization JSONObjectWithData:data options:0 error:&error];
[request setHTTPBody:json];
// [request setValue:[NSString stringWithFormat:#"%d", string.length] forHTTPHeaderField:#"Content-Length"];
NSLog(#"request body:%#", request.HTTPBody);
// NSLog(#"json: %#",json);
// if (!json) {
// // handle error
// NSLog(#"fail");
// }
AFJSONRequestOperation *operation2 = [AFJSONRequestOperation JSONRequestOperationWithRequest:request success:^(NSURLRequest *request, NSHTTPURLResponse *response, id JSON) {
NSLog(#"JSON: %#", JSON);
} failure:nil];
[operation2 start];
That code successfully creates the request body but when it tries to run the block it throws the error and I'm completely stumped. All help would be greatly appreciated.

Answer 1

Never try to build the JSON string yourself like you're doing in the first line. Use NSJSONSerialization to convert from a JSON-compatible Obj-C data structure (like NSDictionary or NSArray) directly to an NSData object to use as the body of the request. For example:
NSDictionary *JSON = [NSDictionary dictionaryWithObject:#"105N14560" forKey:#"code"];
id JSONData = [NSJSONSerialization dataWithJSONObject:JSON options:0 error:error];
You should use the resulting JSONData object for both the HTTPBody of the request as well as the content-length of the request. Here is a complete example:
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:URL]; // URL = wherever the request should be sent to
[request setHTTPMethod:#"POST"];
[request setValue:#"application/json" forHTTPHeaderField:#"Accept"];
[request setValue:#"application/json" forHTTPHeaderField:#"content-type"];
id JSONData = [NSJSONSerialization dataWithJSONObject:JSON options:0 error:error];
if (JSONData) {
[request setValue:[NSString stringWithFormat:#"%d",[(NSData *)JSONData length]] forHTTPHeaderField:#"content-length"];
[request setHTTPBody:JSONData];
}
This just creates the request. The rest of it is straightforward using AFNetworking, where using AFJSONRequestOperation you just pass in the request as you've already done.