Is ServiceLocator an anti-pattern?

Question

If you define patterns as anti-patterns just because there are some situations where it does not fit, then YES it's an anti pattern. But with that reasoning all patterns would also be anti patterns.

Instead we have to look if there are valid usages of the patterns, and for Service Locator there are several use cases. But let's start by looking at the examples that you have given.

public class MyType
{
    public void MyMethod()
    {
        var dep1 = Locator.Resolve<IDep1>();
        dep1.DoSomething();

        // new dependency
        var dep2 = Locator.Resolve<IDep2>();
        dep2.DoSomething();
    }
}

The maintenance nightmare with that class is that the dependencies are hidden. If you create and use that class:

var myType = new MyType();
myType.MyMethod();

You do not understand that it has dependencies if they are hidden using service location. Now, if we instead use dependency injection:

public class MyType
{
    public MyType(IDep1 dep1, IDep2 dep2)
    {
    }

    public void MyMethod()
    {
        dep1.DoSomething();

        // new dependency
        dep2.DoSomething();
    }
}

You can directly spot the dependencies and cannot use the classes before satisfying them.

In a typical line of business application you should avoid the use of service location for that very reason. It should be the pattern to use when there are no other options.

Is the pattern an anti-pattern?

No.

For instance, inversion of control containers would not work without service location. It's how they resolve the services internally.

But a much better example is ASP.NET MVC and WebApi. What do you think makes the dependency injection possible in the controllers? That's right -- service location.

Your questions

But wait a second, if we were using DI approach, we would introduce a dependency with another parameter in constructor (in case of constructor injection). And the problem will be still there.

There are two more serious problems:

1. With service location you are also adding another dependency: The service locator.
2. How do you tell which lifetime the dependencies should have, and how/when they should get cleaned up?

With constructor injection using a container you get that for free.

If we may forget to setup ServiceLocator, then we may forget to add a new mapping in our IoC container and DI approach would have the same run-time problem.

That's true. But with constructor injection you do not have to scan the entire class to figure out which dependencies are missing.

And some better containers also validate all dependencies at startup (by scanning all constructors). So with those containers you get the runtime error directly, and not at some later temporal point.

Also, author mentioned about unit test difficulties. But, won't we have issues with DI approach?

No. As you do not have a dependency to a static service locator. Have you tried to get parallel tests working with static dependencies? It's not fun.

I would also like to point out that IF you are refactoring legacy code that the Service Locator pattern is not only not an anti-pattern, but it is also a practical necessity. No-one is ever going to wave a magic wand over millions of lines of code and suddenly all that code is going to be DI ready. So if you want to start introducing DI to an existing code base it is often the case that you will change things to become DI services slowly, and the code that references these services will often NOT be DI services. Hence THOSE services will need to use the Service Locator in order to get instances of those services that HAVE been converted to use DI.

So when refactoring large legacy applications to start to use DI concepts I would say that not only is Service Locator NOT an anti-pattern, but that it is the only way to gradually apply DI concepts to the code base.

From the testing point of view, Service Locator is bad. See Misko Hevery's Google Tech Talk nice explanation with code examples http://youtu.be/RlfLCWKxHJ0 starting at minute 8:45. I liked his analogy: if you need $25, ask directly for money rather than giving your wallet from where money will be taken. He also compares Service Locator with a haystack that has the needle you need and knows how to retrieve it. Classes using Service Locator are hard to reuse because of it.

Maintenance issue (which puzzles me)

There are 2 different reasons why using service locator is bad in this regard.

1. In your example, you are hard-coding a static reference to the service locator into your class. This tightly couples your class directly to the service locator, which in turns means it won't function without the service locator. Furthermore, your unit tests (and anybody else who uses the class) are also implicitly dependent on the service locator. One thing that has seemed to go unnoticed here is that when using constructor injection you don't need a DI container when unit testing, which simplifies your unit tests (and developers' ability to understand them) considerably. That is the realized unit testing benefit you get from using constructor injection.
2. As for why constructor Intellisense is important, people here seem to have missed the point entirely. A class is written once, but it may be used in several applications (that is, several DI configurations). Over time, it pays dividends if you can look at the constructor definition to understand a class's dependencies, rather than looking at the (hopefully up-to-date) documentation or, failing that, going back to the original source code (which might not be handy) to determine what a class's dependencies are. The class with the service locator is generally easier to write, but you more than pay the cost of this convenience in ongoing maintenance of the project.

Plain and simple: A class with a service locator in it is more difficult to reuse than one that accepts its dependencies through its constructor.

Consider the case where you need to use a service from LibraryA that its author decided would use ServiceLocatorA and a service from LibraryB whose author decided would use ServiceLocatorB. We have no choice other than using 2 different service locators in our project. How many dependencies need to be configured is a guessing game if we don't have good documentation, source code, or the author on speed dial. Failing these options, we might need to use a decompiler just to figure out what the dependencies are. We may need to configure 2 entirely different service locator APIs, and depending on the design, it may not be possible to simply wrap your existing DI container. It may not be possible at all to share one instance of a dependency between the two libraries. The complexity of the project could even be further compounded if the service locators don't happen to actually reside in the same libraries as the services we need - we are implicitly dragging additional library references into our project.

Now consider the same two services made with constructor injection. Add a reference to LibraryA. Add a reference to LibraryB. Provide the dependencies in your DI configuration (by analyzing what is needed via Intellisense). Done.

Mark Seemann has a StackOverflow answer that clearly illustrates this benefit in graphical form, which not only applies when using a service locator from another library, but also when using foreign defaults in services.