如何将依赖项注入到 static class

Question

In my application I regularly want to write log messages to disk. I created a simple logger class and it's constructed using Dependency Injection, as follows:

var logger = new LogService(new FileLogger());
logger.WriteLine("message");

But this now means that every class in my system that needs to log, needs to get this LogService injected into it, which seems redundant. I, therefore, like to make the LogService static instead. This way it doesn't need to be injected into consuming classes.

But here comes the problem. If I make a logger class static, there is no way to inject dependency by constructor into that static class.

So, I changed my LogService to this.

public static class LogService()
{
    private static readonly ILoggable _logger;
    static LogService()
    {
         _logger = new FileLogger();
    }
    
    public static void WriteLine(string message) ...
}

This feels weird to me. I think this is not DI anymore.

What's the best way for me to inject dependencies into a static class?

Answer 1

Dependency Injection, as a practice, is meant to introduce abstractions (or seams ) to decouple volatile dependencies. A volatile dependency is a class or module that, among other things, can contain nondeterministic behavior or in general is something you which to be able to replace or intercept.

For a more detailed discussion about volatile dependencies, see section 1.3.2 of this freely readable introduction of my book .

Because your FileLogger writes to disk, it contains nondeterministic behavior . For this reason you introduced the ILoggable abstraction. This allows consumers to be decoupled from the FileLogger implementation.

To be able to successfully decouple a consumer from its volatile dependency, however, you need to inject that dependency into the consumer. There are three common patterns to choose from:

• Constructor Injection —Dependencies are statically defined as list of parameters to the class's instance constructor.
• Property Injection —Dependencies are injected into the consumer via writable instance properties.
• Method Injection —Dependencies are injected into the consumer as method parameters.

Both Constructor Injection and Property Injection are applied inside the startup path of the application (aka the Composition Root ) and require the consumer to store the dependency in a private field for later reuse. This requires the constructor and property to be instance members , ie non-static. Static constructors can't have any parameters and static properties lead to the Ambient Context anti-pattern (see section 5.3), and Temporal Coupling . This hinders testability and maintainability.

Method injection, on the other hand, is applied outside the Composition Root and it does not store any supplied dependency, but instead merely uses it.

Method injection is, therefore, the only of the three patterns that can be applied to both instance and static methods.

In that case, the method's consumer must supply the dependency. This does mean, however, that the consumer itself must have been supplied with that dependency either through constructor, property, or method injection.

Your example of the static LogService that created FileLogger inside its constructor is a great example of tightly coupled code. This is known as the Control Freak anti-pattern (section 5.1) or in general can be seen as a DIP violation . This is the opposite of DI.

To prevent tight coupling of volatile dependencies, the best is to make LogService non-static and inject its volatile dependencies into its sole public constructor.

Answer 2

It doesn't make sense to use dependency injection (DI) with a static class. Instead of DI, simply add an initialization method to your static class and pass in the dependency.

public static class LogService
{
    private static ILoggable _logger;

    public static ILoggable Logger
    {
        get
        {
             return _logger;
        }
    }

    public static void InitLogger(ILoggable logger)
    {
         _logger = logger;
    }
}

To use the logger, just make sure to call InitLogger() first:

LogService.InitLogger(new FileLogger());
LogService.Logger.WriteLine("message");

Answer 3

You could use Lazy initialization for any object you need to inject to a static class.

https://docs.microsoft.com/en-us/dotnet/api/system.lazy-1?view=net-5.0

This would allow you to pass around static objects that can be shared across running instances and other classes/methods that need to use those objects. An example would be an HttpClient that you want to share across your entire application. You can lazy initialize the HttpClient inside of a static class and refer to the static class to get the HttpClient.

Here's another example using a CosmosDB client: https://docs.microsoft.com/en-us/azure/azure-functions/manage-connections?tabs=csharp#azure-cosmos-db-clients

Answer 4

Here's an example of injecting log4net loggers with Unity DI container using UnityLog4NetResolver extension:

public class A
{
    readonly ILog log;

    public A(ILog log)
    {
        this.log = log;
    }

    ...
}

...

var container = new UnityContainer();

container
    .AddNewExtension<UnityLog4NetResolver.LogByTypeResolverExtension>();

container.Resolve<A>();

The last statement is equivalent to new A(LogManager.GetLogger(typeof(A))) . Thus each class can have a logger with a corresponding name injected by the DI container.

The idea was taken from this package . However, it seems to be abandoned and does not support last Unity version.