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Single responsibility principle - Is it bad to use a single access point for all classes?

I am making a game, in this game I have units. I used to just have one big class called Unit which did more or less everything, which is bad.
So I split it up into several classes, UnitSelectable, UnitAnimator, UnitDetection, UnitAi, UnitHealth, UnitDataContainer, and so on...
Now to the problem, I am using a State Machine where I want states to be quite generic, so they just take a parameter (Unit unit), then I can do whatever I want in the State using this. But now that I have a ton of classes managing the behaviour of a Unit, so what do I pass to my State? What if I want to do something in the state that needs UnitAnimator and UnitHealth?
So to my question, would it be bad to create a class Unit that simply holds a reference to all other classes above, then I can pass that Unit to my state machine and do whatever I want in there?

I am not quite sure what a Unit is in your context, but what about to make your Unit* classes implementing a common interface ( as they are all units probably, and they share some common behavior I assume because of their name) and then pass a parameter of the Unit interface to your State? If you do that you can implement the behavior using polymorphism avoiding conditional code.
"would it be bad to create a class Unit that simply holds a reference to all other classes above..."
Your approach is basically the Mediator pattern where you encapsulate how a set of objects interact, keeping your objects from referring to each other explicitly (in your case the Unit class would hold the references of all the communicating objects). This is not a bad approach at all because you decouple the specific classes from each other, but you have to keep in mind that the Unit class will need to be very intimate with all the different classes, and become really complex (making it hard to maintain).
The best solution from my perspective would be to derive all your Unit* classes from a common interface and so your State machine would just call the appropriate actions polymorphically, but I understand this is not always possible, so going with a Unit Mediator object is another possible solution.

Is this misuse of a singleton?

I've read all about singletons and how they pose the threat of being an anti pattern if misused. So I wanted to get a second opinion as to whether or not this is a misuse of the pattern.
Essentially I have 5 repositories. All they do is store data. In reality, all of the data they store is closely related, I've only made them 5 different repositories so that the classes are short and easy to pick through. I know that if I make each of these repositories a singleton, I can say good bye to any maintainable unit tests; however; I had this idea that I could make each repository a normal class and then make a singleton that simply stores 1 copy of each of the repositories.
This way I can fulfill the requirements of there only being one central location for the data per instance of my program but I can also unit test each repository in some of the operations it needs to perform.

Sure, you may be able to unit-test repositories, but you will certainly have a hard time unit testing all the other code which depends on these repositories. Singleton couples the calling code to the one-and-only implementation through direct access scattered all over your code base. And even if there really is a single copy of your data, i.e. an in-memory database of some kind, there is no reason to let other layers know this.
Also, by "making them a simple class" (I presume you mean, by exposing a public constructor?), you are defeating the whole point of a singleton, i.e. the notion that no other code can instantiate a different instance.
If repositories are a dependency for a certain class, then simply pass them using constructor injection and make your testing easier. This will allow you to easily mock each repository when testing classes from the domain layer.

What constitutes misuse of a singleton (or it's overall usefulness at all) is one of those religious arguments that has no clear-cut answer.
However, you have hit on one of the major drawbacks of the singleton pattern, and that is that it complicates unit tests. Worse, it generally degrades the overall maintainability of your code.
Consider this possible replacement;
public interface IRepositoryContainer
{
IRepository GetRepository<T>() where T : IRepository;
}
public partial class App : Application, IRepositoryContainer
{
private List<IRepository> repositories =
new List<IRepository>() { new MyRespository() };
public IRepository GetRepository<T>()
where T : IRepository
{
return repositories.Where(t => t is T).SingleOrDefault();
}
then where before you would have called a singleton, instead...
IRepository repo = (Application.Current as IRepositoryContainer).GetRepository<MyRespository>();
Just a thought, but there is already only one instance of the application, interfaces can help you solve this problem.
(yes, the example code above is wpf, but the principals should apply)

Is it recommended to mock concrete class?

Most of the examples given in mocking framework website is to mock Interface. Let say NSubstitute that I'm currently using, all their mocking examples is to mock interface.
But in reality, I saw some developer mock concrete class instead. Is it recommended to mock concrete class?

In theory there is absolutely no problem mocking a concrete class; we are testing against a logical interface (rather than a keyword interface), and it does not matter whether that logical interface is provided by a class or interface.
In practice .NET/C# makes this a bit problematic. As you mentioned a .NET mocking framework I'm going to assume you're restricted to that.
In .NET/C# members are non-virtual by default, so any proxy-based methods of mocking behaviour (i.e. derive from the class, and override all the members to do test-specific stuff) will not work unless you explicitly mark the members as virtual. This leads to a problem: you are using an instance of a mocked class that is meant to be completely safe in your unit test (i.e. won't run any real code), but unless you have made sure everything is virtual you may end up with a mix of real and mocked code running (this can be especially problematic if there is constructor logic, which always runs, and is compounded if there are other concrete dependencies to be new'd up).
There are a few ways to work around this.
Use interfaces. This works and is what we advise in the NSubstitute documentation, but has the downside of potentially bloating your codebase with interfaces that may not actually be needed. Arguably if we find good abstractions in our code we'll naturally end up with neat, reusable interfaces we can test to. I haven't quite seen it pan out like that, but YMMV. :)
Diligently go around making everything virtual. An arguable downside to this is that we're suggesting all these members are intended to be extension points in our design, when we really just want to change the behaviour of the whole class for testing. It also doesn't stop constructor logic running, nor does it help if the concrete class requires other dependencies.
Use assembly re-writing via something like the Virtuosity add-in for Fody, which you can use to modify all class members in your assembly to be virtual.
Use a non-proxy based mocking library like TypeMock (paid), JustMock (paid), Microsoft Fakes (requires VS Ultimate/Enterprise, though its predecessor, Microsoft Moles, is free) or Prig (free + open source). I believe these are able to mock all aspects of classes, as well as static members.
A common complaint lodged against the last idea is that you are testing via a "fake" seam; we are going outside the mechanisms normally used for extending code to change the behaviour of our code. Needing to go outside these mechanisms could indicate rigidity in our design. I understand this argument, but I've seen cases where the noise of creating another interface/s outweighs the benefits. I guess it's a matter of being aware of the potential design issue; if you don't need that feedback from the tests to highlight design rigidity then they're great solutions.
A final idea I'll throw out there is to play around with changing the size of the units in our tests. Typically we have a single class as a unit. If we have a number of cohesive classes as our unit, and have interfaces acting as a well-defined boundary around that component, then we can avoid having to mock as many classes and instead just mock over a more stable boundary. This can make our tests a more complicated, with the advantage that we're testing a cohesive unit of functionality and being encouraged to develop solid interfaces around that unit.
Hope this helps.

Update:
3 years later I want to admit that I changed my mind.
In theory I still do not like to create interfaces just to facilitate creation of mock objects. In practice ( I am using NSubstitute) it is much easier to use Substitute.For<MyInterface>() rather than mock a real class with multiple parameters, e.g. Substitute.For<MyCLass>(mockedParam1, mockedParam2, mockedParam3), where each parameter should be mocked separately. Other potential troubles are described in NSubstitute documentation
In our company the recommended practice now is to use interfaces.
Original answer:
If you don't have a requirement to create multiple implementations of the same abstraction, do not create an interface.  
As it pointed by David Tchepak, you don't want to bloating your codebase with interfaces that may not actually be needed.
From http://blog.ploeh.dk/2010/12/02/InterfacesAreNotAbstractions.aspx
Do you extract interfaces from your classes to enable loose
coupling? If so, you probably have a 1:1 relationship between your
interfaces and the concrete classes that implement them.
That’s probably not a good sign, and violates the Reused Abstractions
Principle (RAP).
Having only one implementation of a given interface is a code smell.
If your target is the testability, i prefer  the second option from David Tchepak's answer above.
However I am not convinced that you have to make everything virtual. It's sufficient to make virtual only the methods, that you are going to substitute.
I also will add a comment next to the method declaration that method is virtual only to make it substitutable for unit test mocking.
However note that substitution of concrete classes instead of interfaces has some limitations.
E.g. for NSubstitute
Note: Recursive substitutes will not be created for classes, as
creating and using classes can have potentially unwanted side-effects
.

The question is rather: Why not?
I can think of a couple of scenarios where this is useful, like:
Implementation of a concrete class is not yet complete, or the guy who did it is unreliable. So I mock the class as it is specified and test my code against it.
It can also be useful to mock classes that do things like database access. If you don't have a test database you might want to return values for your tests that are always constant (which is easy by mocking the class).

Its not that it is recommended, it's that you can do this if you have no other choice.
Usually well designed project rely on defining interfaces for your separate components so you can tests each of them in isolation by mocking the other ones. But if you are working with legacy code /code that you are not allowed to change and still want to test your classes then you have no choice and you cannot be criticized for it (assuming you made the effort to try to switch these components to interfaces and were denied the right to).

Supposed we have:
class Foo {
fun bar() = if (someCondition) {
“Yes”
} else {
“No”
}
}
There’s nothing preventing us to do the following mocking in the test code:
val foo = mock<Foo>()
whenever(foo.bar()).thenReturn(“Maybe”)
The problem is it is setting up incorrect behavior of class Foo. The real instance of class Foo will never be able to return “Maybe”.

How do I mock this?

In a .NET windows app, I have a class named EmployeeManager. On instantiation, this class loads employees into a List from the database that haven't completed registration. I'd like to use EmployeeManager in unit test. However, I don't want to involve the database.
From what I understand about this scenario, I need an IEmployeeManager interface, which is only used for testing purposes. This doesn't seem right since the interface has no other use. However, it will allow me to create some EmployeeManager test class that loads employees without involving the database. This way, I can assign values that would have otherwise come from the database.
Is the above correct and do I need to Mock it? Mocking (Moq framework) seems to use lots of code just to do simple things such as assigning a property. I don't get the point. Why mock when I can just create a simple test class from IEmployeeManager that will provide what I need?

Inversion of control is your solution, not Mock objects. Here's why:
You mock the interface to make sure that some code that utilizes your IEmployeeManager is using it properly. You aren't using the test code to prove IEmployeeManager works. So there has to be another class that takes an IEmployeeManager, for instance, which you will actually be testing with your mock object.
If you are actually just testing EmployeeManager, you can do much better. Consider dependency injection. In this manner, you will expose a constructor for EmployeeManager that will take at least one parameter which is an interface. Your EmployeeManager code will internally use this interface for any implementation specific calls that it needs to make.
See Strategy Pattern
This will lead you into a whole, exciting world of Inversion of Control. And as you dig into that, you will find that problems like these have been effectively solved with IoC containers such as AutoFac, Ninject, and Structure Map, to name a few.
Mocking interfaces is great, and you can mock an interface that you then pass into IoC. But you'll find that IoC is a much more robust solution to your problem. And yes, while you might only be implementing a second alternative just for testing, it is still important to do for that very reason -- seperating the strategy under test from the business logic of EmployeeManager.

From what I understand about this scenario, I need an IEmployeeManager interface, which is only used for testing purposes. This doesn't seem right since the interface has no other use.
It's well worth creating the interface. Note also that the interface actually has multiple purposes:
The interface identifies roles or responsibilities provided by an actor. In this case, the interface identifies the roles and responsibilities of the EmployeeManager. By using an interface you're preventing an accidental dependency on something database specific.
The interface reduces coupling. Since your application won't depend on the EmployeeManager, you're free to swap out its implementation without needing to recompile the rest of the application. Of course, this depends on project structure, number of assemblies, etc., but it nevertheless allows this type of reuse.
The interface promotes testability. When you use an interface it becomes much easier to generate dynamic proxies that allow your software to be more easily tested.
The interface forces thought1. Ok, I kind of already alluded to it, but it's worth saying again. Just using an interface alone should make you think about an object's roles and responsibilities. An interface shouldn't be a kitchen sink. An interface represents a cohesive set of roles and responsibilities. If an interface's methods aren't cohesive or aren't almost always used together then it's likely that an object has multiple roles. Though not necessarily bad, it implies that multiple distinct interfaces are better. The larger an interface the harder it is to make it covariant or contravariant and, therefore, more malleable in code.
However, it will allow me to create some EmployeeManager test class that loads employees without involving the database.... I don't get the point. Why mock when I can just create a simple test class from IEmployeeManager that will provide what I need?
As one poster pointed out, it sounds like you're talking about creating a stub test class. Mocking frameworks can be used to create stubs, but one of the most important features about them is that they allow you to test behavior instead of state. Now let's look at some examples. Assume the following:
interface IEmployeeManager {
void AddEmployee(ProspectiveEmployee e);
void RemoveEmployee(Employee e);
}
class HiringOfficer {
private readonly IEmployeeManager manager
public HiringOfficer(IEmployeeManager manager) {
this.manager = manager;
}
public void HireProspect(ProspectiveEmployee e) {
manager.AddEmployee(e);
}
}
When we test the HiringOfficer's HireEmployee behavior, we're interested in validating that he correctly communicated to the employee manager that this perspective employee be added as an employee. You'll often see something like this:
// you have an interface IEmployeeManager and a stub class
// called TestableEmployeeManager that implements IEmployeeManager
// that is pre-populated with test data
[Test]
public void HiringOfficerAddsProspectiveEmployeeToDatabase() {
var manager = new TestableEmployeeManager(); // Arrange
var officer = new HiringOfficer(manager); // BTW: poor example of real-world DI
var prospect = CreateProspect();
Assert.AreEqual(4, manager.EmployeeCount());
officer.HireProspect(prospect); // Act
Assert.AreEqual(5, manager.EmployeeCount()); // Assert
Assert.AreEqual("John", manager.Employees[4].FirstName);
Assert.AreEqual("Doe", manager.Employees[4].LastName);
//...
}
The above test is reasonable... but not good. It's a state-based test. That is, it verifies the behavior by checking the state before and after some action. Sometimes this is the only way to test things; sometimes it's the best way to test something.
But, testing behavior is often better, and this is where mocking frameworks shine:
// using Moq for mocking
[Test]
public void HiringOfficerCommunicatesAdditionOfNewEmployee() {
var mockEmployeeManager = new Mock<EmployeeManager>(); // Arrange
var officer = new HiringOfficer(mockEmployeeManager.Object);
var prospect = CreateProspect();
officer.HireProspect(prospect); // Act
mockEmployeeManager.Verify(m => m.AddEmployee(prospect), Times.Once); // Assert
}
In the above we tested the only thing that really mattered -- that the hiring officer communicated to the employee manager that a new employee needed to be added (once, and only once... though I actually wouldn't bother checking the count in this case). Not only that, I validated that the employee that I asked the hiring officer to hire was added by the employee manager. I've tested the critical behavior. I didn't need even a simple test stub. My test was shorter. The actual behavior was much more evident -- it becomes possible to see the interaction and validate interaction between objects.
It is possible to make your stub test class record interactions, but then you're emulating the mocking frameworks. If you're going to test behavior -- use a mocking framework.
As another poster mentioned, dependency injection (DI) and inversion of control (IoC) are important. My example above isn't a good example of this, but both should be carefully considered and judiciously used. There's a lot of writing on the subject available.
1 - Yes, thinking is still optional, but I'd strongly recommend it ;).

Creating an IEmployeeManager interface in order to be able to mock is that way most .NET developers would go about making such a class testable.
Another option is to inherit from EmployeeManager and override the method you want to test so it will not involve the database - this too means you will need to change your design.

Extracting interface in your scenario is a good idea. I would not worry too much about the fact that you only need this for testing. Extracting this interface makes your code decoupled from database. After that you will have a choice between writing your own implementation for testing or use mocking framework to generate this implementation for you. This is a matter of personal preference. It depends on how familiar you are with mocking framework and whether you want to spend time learning new syntax.
In my opinion it is worth learning. It will save you a lot of typing. They are also flexible and don't always require an interface to generate test implementation. RhinoMocks for example can mock concrete classes as long they have empty constructor and methods are virtual. Another advantage is that mocking APIs use consistent naming so you will get familiar with 'Mocks', 'Stubs' etc. In your scenario by the way you need stub, not mock. Writing an actual mock manually may be more labor intensive than using framework.
The danger with mocking frameworks is that some of them are so powerful and can mock pretty much anything, including private fields (TypeMock). In other words they are too forgiving to design mistakes and allow you to write very coupled code.
This is a good read on the subject of hand written vs. generated stubs

By making your classes implement Interfaces you are not only making them more testable, you're making your application more flexible and maintainable. When you say "This doesn't seem right since the interface has no other use", is flawed since it allows you to loosely couple your classes.
If I could suggest a couple of books Head First Design Patterns and Head First Software Development will do a much better job of explaining the concepts then I could in a SO answer.
If you don't want to use a mocking framework like Moq, it's simple enough to roll your own mock/stubs, here is a quick blog post on it Rolling your own Mock Objects

What do programmers mean when they say, "Code against an interface, not an object."?

I've started the very long and arduous quest to learn and apply TDD to my workflow. I'm under the impression that TDD fits in very well with IoC principles.
After browsing some of TDD tagged questions here in SO, I read it's a good idea to program against interfaces, not objects.
Can you provide simple code examples of what this is, and how to apply it in real use cases? Simple examples is key for me (and other people wanting to learn) to grasp the concepts.

Consider:
class MyClass
{
//Implementation
public void Foo() {}
}
class SomethingYouWantToTest
{
public bool MyMethod(MyClass c)
{
//Code you want to test
c.Foo();
}
}
Because MyMethod accepts only a MyClass, if you want to replace MyClass with a mock object in order to unit test, you can't. Better is to use an interface:
interface IMyClass
{
void Foo();
}
class MyClass : IMyClass
{
//Implementation
public void Foo() {}
}
class SomethingYouWantToTest
{
public bool MyMethod(IMyClass c)
{
//Code you want to test
c.Foo();
}
}
Now you can test MyMethod, because it uses only an interface, not a particular concrete implementation. Then you can implement that interface to create any kind of mock or fake that you want for test purposes. There are even libraries like Rhino Mocks' Rhino.Mocks.MockRepository.StrictMock<T>(), which take any interface and build you a mock object on the fly.

It's all a matter of intimacy. If you code to an implementation (a realized object) you are in a pretty intimate relationship with that "other" code, as a consumer of it. It means you have to know how to construct it (ie, what dependencies it has, possibly as constructor params, possibly as setters), when to dispose of it, and you probably can't do much without it.
An interface in front of the realized object lets you do a few things -
For one you can/should leverage a factory to construct instances of the object. IOC containers do this very well for you, or you can make your own. With construction duties outside of your responsibility, your code can just assume it is getting what it needs. On the other side of the factory wall, you can either construct real instances, or mock instances of the class. In production you would use real of course, but for testing, you may want to create stubbed or dynamically mocked instances to test various system states without having to run the system.
You don't have to know where the object is. This is useful in distributed systems where the object you want to talk to may or may not be local to your process or even system. If you ever programmed Java RMI or old skool EJB you know the routine of "talking to the interface" that was hiding a proxy that did the remote networking and marshalling duties that your client didn't have to care about. WCF has a similar philosophy of "talk to the interface" and let the system determine how to communicate with the target object/service.
** UPDATE **
There was a request for an example of an IOC Container (Factory). There are many out there for pretty much all platforms, but at their core they work like this:
You initialize the container on your applications startup routine. Some frameworks do this via config files or code or both.
You "Register" the implementations that you want the container to create for you as a factory for the interfaces they implement (eg: register MyServiceImpl for the Service interface). During this registration process there is typically some behavioral policy you can provide such as if a new instance is created each time or a single(ton) instance is used
When the container creates objects for you, it injects any dependencies into those objects as part of the creation process (ie, if your object depends on another interface, an implementation of that interface is in turn provided and so on).
Pseudo-codishly it could look like this:
IocContainer container = new IocContainer();
//Register my impl for the Service Interface, with a Singleton policy
container.RegisterType(Service, ServiceImpl, LifecyclePolicy.SINGLETON);
//Use the container as a factory
Service myService = container.Resolve<Service>();
//Blissfully unaware of the implementation, call the service method.
myService.DoGoodWork();

When programming against an interface you will write code that uses an instance of an interface, not a concrete type. For instance you might use the following pattern, which incorporates constructor injection. Constructor injection and other parts of inversion of control aren't required to be able to program against interfaces, however since you're coming from the TDD and IoC perspective I've wired it up this way to give you some context you're hopefully familiar with.
public class PersonService
{
private readonly IPersonRepository repository;
public PersonService(IPersonRepository repository)
{
this.repository = repository;
}
public IList<Person> PeopleOverEighteen
{
get
{
return (from e in repository.Entities where e.Age > 18 select e).ToList();
}
}
}
The repository object is passed in and is an interface type. The benefit of passing in an interface is the ability to 'swap out' the concrete implementation without changing the usage.
For instance one would assume that at runtime the IoC container will inject a repository that is wired to hit the database. During testing time, you can pass in a mock or stub repository to exercise your PeopleOverEighteen method.

It means think generic. Not specific.
Suppose you have an application that notify the user sending him some message. If you work using an interface IMessage for example
interface IMessage
{
public void Send();
}
you can customize, per user, the way they receive the message. For example somebody want to be notified wih an Email and so your IoC will create an EmailMessage concrete class. Some other wants SMS, and you create an instance of SMSMessage.
In all these case the code for notifying the user will never be changed. Even if you add another concrete class.

The big advantage of programming against interfaces when performing unit testing is that it allows you to isolate a piece of code from any dependencies you want to test separately or simulate during the testing.
An example I've mentioned here before somewhere is the use of an interface to access configuration values. Rather than looking directly at ConfigurationManager you can provide one or more interfaces that let you access config values. Normally you would supply an implementation that reads from the config file but for testing you can use one that just returns test values or throws exceptions or whatever.
Consider also your data access layer. Having your business logic tightly coupled to a particular data access implementation makes it hard to test without having a whole database handy with the data you need. If your data access is hidden behind interfaces you can supply just the data you need for the test.
Using interfaces increases the "surface area" available for testing allowing for finer grained tests that really do test individual units of your code.

Test your code like someone who would use it after reading the documentation. Do not test anything based on knowledge you have because you have written or read the code. You want to make sure that your code behaves as expected.
In the best case you should be able to use your tests as examples, doctests in Python are a good example for this.
If you follow these guidelines changing the implementation shouldn't be an issue.
Also in my experience it is good practice to test each "layer" of your application. You will have atomic units, which in itself have no dependencies and you will have units which depend on other units until you eventually get to the application which in itself is a unit.
You should test each layer, do not rely on the fact that by testing unit A you also test unit B which unit A depends on (the rule applies to inheritance as well.) This, too, should be treated as an implementation detail, even though you might feel as if you are repeating yourself.
Keep in mind that once written tests are unlikely to change while the code they test will change almost definitely.
In practice there is also the problem of IO and the outside world, so you want to use interfaces so that you can create mocks if necessary.
In more dynamic languages this is not that much of an issue, here you can use duck typing, multiple inheritance and mixins to compose test cases. If you start disliking inheritance in general you are probably doing it right.

This screencast explains agile development and TDD in practice for c#.
By coding against an interface means that in your test, you can use a mock object instead of the real object. By using a good mock framework, you can do in your mock object whatever you like.