TDD: Static methods, dependency injection, caching and you!

Question

I hope I can explain this a bit decently, as today it blows up a fuse in my brain. I am learning TDD in C #, so I'm still trying to remake my brain to fit it.

Say I have a User class that previously had a static method for retrieving a User object (simplified below).

public static User GetUser(string username)
{
   User user = GetUserFromCache(username);
   if(user == null)
   {
       user = GetUserFromDatabase(username);
       StoreObjectInCache(user);
   }
   return user;
}

So I'm trying to rewrite this to use dependency injection so that I can fake the GetUserFromDatabase method if it should go there. This means that I have to make the function non-static. While the data access layer would create a user object from the database, matching returned columns with object properties, retrieving from the cache would return a real blue User object . However, in the non-stationary method, I can’t just say

this = GetUserFromCache(username);

Because it just doesn't work. Although I am by no means a global expert on how to dance around this with OO, it seems I almost needed to grab the User object from the cache and write another mapping function that would save the returned properties of the user object in a new User instance.

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Answer 1

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Answer 2

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public static Func<string, UserName> Loader {get;set;}

public static Constructor()
{
  Loader = GetFromDataBase;
}

public static User GetUser(string userName)
{
  User user = GetUserFromCache()
  if (user == null)
  {
    user = Loader(userName);
    StoreUserInCache(user);
  }
  return user;
}    

public void Test1()
{
  UserGetter.Loader = Mock.GetUser;
  UserGetter.GetUser("Bob");
}

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Answer 3

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, :

class ApplicationFacade{
  private IUserRepository users = null;

  public doStuff(){
    this.users.GetUser("my-name");
  }
}

IUserRepository - , . - :

interface IUserRepository{
  User GetUser(string username);
}

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class Cache : IUserRepository {
  private IUserRepository users = null;
  public User GetUser(string username){
    if (this.NotCached(username)){
      this.ToCache(this.users.GetUser(username));
    }
    return this.FromCache(username);
  }
}

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Due to the fact that the actual injection mechanism depends on your DI container, some additional code may be required as public properties or constructor fields.

Answer 4

See Refactoring a static method / static field for testing.

The proposed approach may work for you if for some reason you cannot reorganize everything to separate the problems, as suggested in another answer.