Class design and object references

Here is a very basic example of what I am trying to do. In reallity there are more relations but not something extreme or memory heavy.

  public class ClassA : ISomething
        {     
            public double property { get; set; }
            ...
            public ClassB classb { get; set; }
        }

     public class ClassB : ISomething2
        {     
            public double Length { get; set; }
            ...

        }



     public class MyProject : BaseProject
        {     
            public IEnumerable<ISomething> ISomethings { get; set; }
            ....
            public IEnumerable<ISomething2> ISomethings2 { get; set; }
            ...

        }

The issue here is that I have to keep a List of ISomethings2 and then classb property of ClassA to reference only an existing item in ISomething2 list. The hard thing here is that :

• Removing an item from ISomething2 should remove all references to it (classb property of ClassA instances to be set to null)


• Prevent other developers setting classb property to a non existing object in list or a new user created object.


• Allow easy serialization via JSON.NET etc


• Unique id generation


• These should apply also for eg: ISomething3 etc.. even ISomething if it has a reference to another object of type ISomething

These classes are used for project description not database stuff. Like when you open a project file for an application. So changing a property in classB should be visible to all since it is the same object reference. Is there a pattern to achieve what I want without much coupling ? Alternative designs/approaches are also welcome.