http://stackoverflow.com/questions/4344074/nscoding-of-nsmutabledictionaries-containing-custom-objects

test.m

#import <Foundation/Foundation.h>


#define kPreviousSearchEntityKey @"PreviousSearchEntity"
#define kPreviousSearchKey @"PreviousSearch"

#define kIDKey @"ID"
#define kTextKey @"Text"
#define kCategoryKey @"Category"
#define kCategoryIdentifierKey @"CategoryIdentifierKey"


@interface SearchParameterEntity : NSObject

@property (nonatomic, retain) NSMutableDictionary *parameters;

@end

@implementation SearchParameterEntity

@synthesize parameters;

- (void) encodeWithCoder:(NSCoder*)encoder 
{
    //encode all the values so they can be persisted in NSUserdefaults
    if (parameters) 
        [encoder encodeObject:parameters forKey:kPreviousSearchEntityKey]; //GDB:Interrupted!
}

- (id) initWithCoder:(NSCoder*)decoder 
{
    if (self = [super init]) 
    {
        //decode all values to return an object from NSUserdefaults in the same state as when saved     
        [self setParameters:[decoder decodeObjectForKey:kPreviousSearchEntityKey]];
    }
    return self;
}

- (void) dealloc {
    [parameters release];
    [super dealloc];
}

@end


@interface CategoryEntity : NSObject

@property (nonatomic, retain) NSString *ID;
@property (nonatomic, retain) NSString *text;
@property (nonatomic, retain) NSString *category;
@property (nonatomic, retain) NSString *categoryIdentifierKey;

@end

@implementation CategoryEntity

@synthesize ID;
@synthesize text;
@synthesize category;
@synthesize categoryIdentifierKey;

- (void) encodeWithCoder:(NSCoder*)encoder 
{
    //encode all the values so they can be persisted in NSUserdefaults
    [encoder encodeObject:ID forKey:kIDKey];
    [encoder encodeObject:text forKey:kTextKey];
    [encoder encodeObject:category forKey:kCategoryKey];
    [encoder encodeObject:categoryIdentifierKey forKey:kCategoryIdentifierKey];
}

- (id) initWithCoder:(NSCoder*)decoder 
{
    if (self = [super init]) {

        //decode all values to return an object from NSUserdefaults in the same state as when saved
        [self setID:[decoder decodeObjectForKey:kIDKey]];
        [self setText:[decoder decodeObjectForKey:kTextKey]];
        [self setCategory:[decoder decodeObjectForKey:kCategoryKey]];
        [self setCategoryIdentifierKey:[decoder decodeObjectForKey:kCategoryIdentifierKey]];
    }
    return self;
}

- (void) dealloc {
    [ID release];
    [text release];
    [category release];
    [categoryIdentifierKey release];
    [super dealloc];
}

@end


@interface NSUserDefaultsWrapper : NSUserDefaults

@end

@implementation NSUserDefaultsWrapper

+ (id) defaults {
    return [self standardUserDefaults];
}

+ (void) setPreviousSearchParameters:(SearchParameterEntity*) entity
{
    if (entity) 
    {
        //first encode the entity (implements the NSCoding protocol) then save it
        NSData *encodedObject = [NSKeyedArchiver archivedDataWithRootObject:entity];
        NSLog(@"setting defaults, encodedObject = %@", encodedObject);
        [[self defaults] setObject:encodedObject forKey:kPreviousSearchKey];
        [[self defaults] synchronize];      
    }
}

+ (SearchParameterEntity*) getPreviousSearchParameters
{
    //retrieve the encoded NSData object that was saved, decode and return it
    SearchParameterEntity *entity = nil;
    NSData *encodedObject = [[self defaults] objectForKey:kPreviousSearchKey];
    NSLog(@"reading defaults, encodedObject = %@", encodedObject);

    if (encodedObject)
        entity = [NSKeyedUnarchiver unarchiveObjectWithData:encodedObject];

    return entity;
}

@end


void describe(SearchParameterEntity *entity) {

    NSMutableDictionary *parameters = entity.parameters;
    NSLog(@"parameters = %@", parameters);

    CategoryEntity *category = [parameters objectForKey:@"category"];
    NSLog(@"ID = '%@'", category.ID);
    NSLog(@"text = '%@'", category.text);
    NSLog(@"category = '%@'", category.category);
    NSLog(@"categoryIdentifierKey = '%@'", category.categoryIdentifierKey);

}

int main(int argc, char *argv[]) {
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    CategoryEntity *category = [[CategoryEntity alloc] init];    
    category.ID = @"1";
    category.text = @"some text";
    category.category = @"category name";
    category.categoryIdentifierKey = @"category identifier key";

    NSMutableDictionary *parameters = [NSMutableDictionary dictionaryWithObject:category forKey:@"category"];
    
    SearchParameterEntity *searchParameters = [[SearchParameterEntity alloc] init];
    searchParameters.parameters = parameters;

    NSLog(@"persisting entity: %@", searchParameters);
    describe(searchParameters);
    [NSUserDefaultsWrapper setPreviousSearchParameters:searchParameters];

    SearchParameterEntity *restored = [NSUserDefaultsWrapper getPreviousSearchParameters];
    NSLog(@"restored entity: %@", restored);
    describe(restored);

    [pool release];
    return 0;
}