IOS NSMethodSignature stays nil

IOS NSMethodSignature stays nil

I recently started to develop for IOS and struggle with the Invocation
Object.
What i have is a Class "Location", derived from NSManagedObject (it's part
of the Coredata model)
@interface Location (CoreDataGeneratedAccessors)
- (void)addHasLocationInfoObject:(Info *)value;
...
@end
I also have other Classes that have similar signatures
(addHasWorkorderInfoObject,...).
These InfoObjects have constraints with their "parent objects" in this
case "Location" has several "LocationInfo" Objects, which i retrieve from
a Database and want to add to the Location. Same thing should happen to
all Objects that have InfoObjects assigned.
I am now trying to create a method that will work for any Object that
sticks to the naming-conventions fixed by the project-documentation
(Location -> addHasLocationInfoObject, XY -> addHasXYInfoObject ...).
My Approach for adding Infos to Objects now is:
-(void)setInfoForObject:(NSManagedObject *)managedObject
withClass:(NSString *)className
NSString *noteRefName = [[NSString alloc]init];
noteRefName = [NSString stringWithFormat:@"%@Info", className];
NSString *addInfoSelectorName = [[NSString alloc]init];
addInfoSelectorName = [NSString stringWithFormat:@"addHas%@Object::",
infoClassName];
SEL addInfoPropertySelector = NSSelectorFromString(addInfoSelectorName);
NSMethodSignature *signature = [[managedObject class]
methodSignatureForSelector:addNotePropertySelector];
NSInvocation *invocation = [NSInvocation
invocationWithMethodSignature:signature];
[invocation setTarget:managedObject];
[invocation setSelector:addInfoPropertySelector];
[invocation setArgument:&note atIndex:2];
However this gives me an Error, because the Signature object gets set to
nil. I tried searching for the Problem and it seems to be related to the
":" in the Selectors' name.
But I do not understand how many and where and why i have to set these? I
also seem to fail to find the Docs page that tells me how to do it
properly.
Anny help appreciated, Thanks in advance!
PS. I Logged the SelectorName and the Classname and they both are spelled
correctly.