Unit testing with singletons

Question

I have prepared some automatic tests with the Visual Studio Team Edition testing framework. I want one of the tests to connect to the database following the normal way it is

Answer 1

Example from Book: Working Effectively with Legacy Code

Also given same answer here: https://stackoverflow.com/a/28613595/929902

To run code containing singletons in a test harness, we have to relax the singleton property. Here’s how we do it. The first step is to add a new static method to the singleton class. The method allows us to replace the static instance in the singleton. We’ll call it setTestingInstance.

public class PermitRepository
{
    private static PermitRepository instance = null;
    private PermitRepository() {}
    public static void setTestingInstance(PermitRepository newInstance)
    {
        instance = newInstance;
    }
    public static PermitRepository getInstance()
    {
        if (instance == null) {
            instance = new PermitRepository();
        }
        return instance;
    }
    public Permit findAssociatedPermit(PermitNotice notice) {
    ...
    }
    ...
}

Now that we have that setter, we can create a testing instance of a PermitRepository and set it. We’d like to write code like this in our test setup:

public void setUp() {
    PermitRepository repository = PermitRepository.getInstance();
    ...
    // add permits to the repository here
    ...
    PermitRepository.setTestingInstance(repository);
}

Answer 2

You can use constructor dependency injection. Example:

public class SingletonDependedClass
{
    private string _ProviderName;

    public SingletonDependedClass()
        : this(ConfigurationManager.ConnectionStrings["main_db"].ProviderName)
    {
    }

    public SingletonDependedClass(string providerName)
    {
        _ProviderName = providerName;
    }
}

That allows you to pass connection string directly to object during testing.

Also if you use Visual Studio Team Edition testing framework you can make constructor with parameter private and test the class through the accessor.

Actually I solve that kind of problems with mocking. Example:

You have a class which depends on singleton:

public class Singleton
{
    public virtual string SomeProperty { get; set; }

    private static Singleton _Instance;
    public static Singleton Insatnce
    {
        get
        {
            if (_Instance == null)
            {
                _Instance = new Singleton();
            }

            return _Instance;
        }
    }

    protected Singleton()
    {
    }
}

public class SingletonDependedClass
{
    public void SomeMethod()
    {
        ...
        string str = Singleton.Insatnce.SomeProperty;
        ...
    }
}

First of all SingletonDependedClass needs to be refactored to take Singleton instance as constructor parameter:

public class SingletonDependedClass
{    
    private Singleton _SingletonInstance;

    public SingletonDependedClass()
        : this(Singleton.Insatnce)
    {
    }

    private SingletonDependedClass(Singleton singletonInstance)
    {
        _SingletonInstance = singletonInstance;
    }

    public void SomeMethod()
    {
        string str = _SingletonInstance.SomeProperty;
    }
}

Test of SingletonDependedClass (Moq mocking library is used):

[TestMethod()]
public void SomeMethodTest()
{
    var singletonMock = new Mock<Singleton>();
    singletonMock.Setup(s => s.SomeProperty).Returns("some test data");
    var target = new SingletonDependedClass_Accessor(singletonMock.Object);
    ...
}

Answer 3

You are facing a more general problem here. If misused, Singletons hinder testabiliy.

I have done a detailed analysis of this problem in the context of a decoupled design. I'll try to summarize my points:

1. If your Singleton carries a significant global state, don’t use Singleton. This includes persistent storage such as Databases, Files etc.
2. In cases, where dependency on a Singleton Object is not obvious by the classes name, the dependency should be injected. The need to inject Singleton Instances into classes proves a wrong usage of the pattern (see point 1).
3. A Singleton’s life-cycle is assumed to be the same as the application’s. Most Singleton implementations are using a lazy-load mechanism to instantiate themselves. This is trivial and their life-cycle is unlikely to change, or else you shouldn’t use Singleton.

Answer 4

Have a look at the Google Testing blog:

• Using dependency injection to avoid singletons
• Singletons are Pathological Liars
• Root Cause of Singletons
• Where have all the Singletons Gone?
• Clean Code Talks - Global State and Singletons
• Dependency Injection.

And also:

• Once Is Not Enough
• Performant Singletons

Finally, Misko Hevery wrote a guide on his blog: Writing Testable Code.