Adding multiple strings to a property NSMutableString

You are sending the message appendString to nil. Change NSMutableString *theNames = nil; to NSMutableString *theNames = [[NSMutableString alloc] init];

- (NSMutableString *)setNames
{
    NSMutableString *theNames = [[NSMutableString alloc] init];
    [theNames appendString:Name_1];
    [theNames appendString:Name_2];
    [theNames appendString:Name_3];
    return theNames;
}

I'm sorry I did not make that clear. Yes, I have a MutableString property 'names', which is to receive the names from the 3 strings.

the instance variable in which the property 'Names' should be stored. I don't know why it is causing the error

If your error is "No visible @interface for NSMutableString declares the selector", then it sounds like you're missing a -(NSMutableString *)setNames; declaration in your class's .h file.

Another thing you could try is calling setNames on self, rather than _Names, since setNames is a method of your object, not of NSMutableString. So instead of self.Names = [_Names setNames]; try self.Names = [self setNames];, which should return your NSMutableString theNames, and then set self.Names to it.

And Chris is right too, make sure you alloc and init your theNames object before calling appendString on it!

...And one other teeny thing is that the convention, as far as I can tell, is to use lowercase instance variable names (so "names", not "Names").

Hope this helps—good luck!

-csj