How to solve this error? "property with 'retain(or strong)' attribute must be of object type"

Question

I have been working on an app all day which has been working fine until Xcode downloaded libraries or something and then it started having issues. I am just trying to create the getter/setter methods to get a couple of arrays out of my APPDelegate. Like I said it was working fine and then randomly it showed up with this error and now won't build anymore:

property with 'retain(or strong)' attribute must be of object type

Here is the rest of the code:

#import <Foundation/Foundation.h>
#import "Project.h"
#import "TeamMember.h"


@interface Task : NSObject{

NSDate *endDate;
NSDate *startDate;
NSMutableString* notes;
NSMutableString* taskName;

//The error appears first right here over teamMember
TeamMember *teamMember;
Project *project;

}
//The error appears over both of the following lines as well...
@property  (nonatomic, retain)TeamMember *teamMember;
@property  (nonatomic, retain) Project * project;

@property (nonatomic, retain) NSMutableString *notes;
@property (nonatomic, retain) NSMutableString *taskName;
@property (nonatomic, retain) NSDate *startDate;
@property (nonatomic, retain) NSDate *endDate;


@end

Any ideas? This has got me stumped....

Here is Project.h:

#import <Foundation/Foundation.h>
#import "Task.h"

@interface Project : NSObject{

NSDate *dueDate;
NSDate *startDate;
NSArray *tasksInProject;
NSMutableString* notes;
NSMutableString* description;
NSMutableString* projectName; 

}
@property (nonatomic, retain) NSDate *startDate;
@property (nonatomic, retain) NSDate *dueDate;
@property (nonatomic, retain) NSArray *tasksInProject;
@property (nonatomic, retain) NSMutableString *description;
@property (nonatomic, retain) NSMutableString *projectName;
@end

Here is TeamMember.h

#import <Foundation/Foundation.h>
#import "Task.h"
#import "Project.h"

@interface TeamMember : NSObject{

NSMutableArray *projects;
NSMutableString *name;
NSMutableString *title;
NSMutableString *email;
NSMutableString *phone;
NSMutableString *notes;
}

//@property(nonatomic, retain) NSArray *projects;
@property (nonatomic, retain) NSMutableString *name;
@property (nonatomic, retain) NSMutableString *title;
@property (nonatomic, retain) NSMutableString *email;
@property (nonatomic, retain) NSMutableString *phone;
@property (nonatomic, retain) NSMutableString *notes;
@end

Answer 1

It looks like it's caused by recursively including of header files.

Try to add @class Project and @class TeamMember into your Task.h, like this

#import <Foundation/Foundation.h>
#import "Project.h"
#import "TeamMember.h"

@class TeamMember;
@class Project;

@interface Task : NSObject{
    NSDate *endDate;
    NSDate *startDate;
    ...
}
@end