How to check if an NSDictionary or NSMutableDictionary contains a key?

objectForKey will return nil if a key doesn't exist.

if ([[dictionary allKeys] containsObject:key]) {
    // contains key
}

or

if ([dictionary objectForKey:key]) {
    // contains object
}

More recent versions of Objective-C and Clang have a modern syntax for this:

if (myDictionary[myKey]) {

}

You do not have to check for equality with nil, because only non-nil Objective-C objects can be stored in dictionaries(or arrays). And all Objective-C objects are truthy values. Even @NO, @0, and [NSNull null] evaluate as true.

Edit: Swift is now a thing.

For Swift you would try something like the following

if let value = myDictionary[myKey] {

}

This syntax will only execute the if block if myKey is in the dict and if it is then the value is stored in the value variable. Note that this works for even falsey values like 0.

if ([mydict objectForKey:@"mykey"]) {
    // key exists.
}
else
{
    // ...
}

When using JSON dictionaries:

#define isNull(value) value == nil || [value isKindOfClass:[NSNull class]]

if( isNull( dict[@"my_key"] ) )
{
    // do stuff
}