How to fetch data based on id from isolated code cleanest way?

Question

I'm working on client-server application. Client architecture (class hierarchy) looks like this:

• System (name is of course different, more descriptive) - represents whole system
  • Module A - a class instance representing one module. One module = group of similar data and operations
  • User Module - another module
    • Users - class instance which represents collection of users, allows adding, removing, reading, etc.
    • Groups - represents user groups
  • etc.

Architecture above represents the system, data, operations. Handles client requests and server responses. Caches queries if required. But it does NOT handle GUI, for that I use separated code. The most of the gui uses view for displaying data which is based on simple logic: one view (class) type represents a view of one (single) item type. For example UserView displays data about user (from User instance).

Often the item represents a collection of items. For example there might be item like UserEvent which contains some name (of the event), date and list of users. However, the list of users is implemented as list of IDs (UserId), nothing more. So now UserEventView takes this list and wants to create instances of UserView in order to display more detailed information about users (for example as list of name). So now the UserEventView needs to access Users to translate UserId to User in order to provide UserView correct data. So, what's the best way for the UserEventView to access Users?

I can see multiple ways how to solve it, but none seems to be ideal:

• Singleton - the most simple and easiest way. Either of System (even though UserEvenView doesn't care about it) or for every single module.
• Users instance is given to UserEventView by it's parent, which is provided by it's parent, which is provided by it's parent, ...
• There is UsersProvider component attached to the UserView which searches for GUI root where it searches for a component which provides System instance and obtains Users from it.

Notes:

• If you want to suggest some exception in design for users, then imagine I'm working with cars, shoes, anything.
• The app is written in C#/C++.

Answer 1

Create a repository to wrap whatever stores you users with appropriate methods

IUserRepository
{
    GetUserById(string id);
}

Instanciate a concrete instance of this at the root of your app

class System
{
    public void Main(string[] args)
    {
        var repo = new MyUserRepo();
        ....
    }
}

and pass it into object that need it via construction parameters. ie.

class UserViewModel
{
    public UserViewModel(IUserRepository repo)
}

Now your various ViewModels and/or Controllers etc can get Users without being tightly coupled to the rest of the App.

Note: Although this pattern is usually used to wrap a database, theres no reason why you cant wrap a List or Dictionary of in memory Users instead