Category that enforces objects in a dictionary are of the type I expect them to be.

foo.h

/**

THE PROBLEM:

NSDictionary *d = [sever getMeta];
// d.score returns something of type 'NSArray'

NSNumber *userId = [d objectForKey:@"score"];
NSInteger i = [userId integerValue];    // <<------- Now what happens!



SOLUTION: 

NSDictionary *d = [sever getMeta];

//Raises exception in DEBUG builds immediately if the Class is not a match
NSNumber *userId = [d objectForKey:@"score" ofClass:@"NSNumber"]; 

*/

@interface NSObject (AssertionCategories)

- (id)objectForKey:(id)aKey ofClass:(NSString *)aClassName;
- (id)objectForKey:(id)aKey ofClass:(NSString *)aClassName mustExist:(BOOL)mustExist;

@end

foo.m

@implementation NSObject (AssertionCategories)

- (id)objectForKey:(id)aKey ofClass:(NSString *)aClassName;
{
    return [self objectForKey:aKey ofClass:aClassName mustExist:YES];
}

- (id)objectForKey:(id)aKey ofClass:(NSString *)aClassName mustExist:(BOOL)mustExist {
    
    id object = [(NSDictionary *)self objectForKey:aKey];
    
#ifdef DEBUG
    
    // if object exists, check the class
    // if it doesn't, check whether mustExist is YES
    // if it isn't, return the object (nil)
    
    if (object) {
        // cannot include this string directly in NSCAssert; but very helpful for debugging
        NSString *errorString = [NSString stringWithFormat:@"Invalid kind of class; expected: %@, actual: %@",
                                 aClassName,
                                 NSStringFromClass([object class])];
        
        NSCAssert([object isKindOfClass:NSClassFromString(aClassName)] == YES,errorString);
    } else if (mustExist) {
        
        NSString *errorString = @"Object does not exist!";
        NSCAssert(NO,errorString);
        
    }
#endif
    
    return object;
    
}

@end