Solve real coding problems
Articles about solving coding problems with the Dependency Injection technique.
If your code does File IO, such as reading text from a file, then it’s dependent on the file system. This is an external dependency. In order to make the unit tests fast and reliable, you can mock out the external dependencies. To mock out the file system dependency, you can wrap the File IO … Read more
You can use Microsoft Extensibility Framework (MEF) to load assemblies at runtime. This is an alternative to implementing dynamic assembly loading with a more manual approach (like using AssemblyLoadContext). Here’s an example of using MEF to load an instance of IMessageProcessorPlugin from some assembly located in the C:\Plugins directory: MEF looks for exported types in … Read more
When you’re unit testing code that uses the console IO methods, you’ll need a way to automate the input and a way to capture the output. There are two ways to do this: Redirect the console IO. Wrap the console IO method calls and mock out the wrapper. In this article, I’ll show how to … Read more
I was reading about logging in ASP.NET when I came across this statement about logging to the database: When logging to SQL Server, don’t do so directly. Instead, add log messages to an in-memory queue and have a background worker dequeue and insert data to SQL Server. Paraphrased from Microsoft – No asynchronous logger methods … Read more
When a background service throws an exception in ExecuteAsync() and stops running, nothing happens. No errors are logged, and the ASP.NET service continues to run. What if you want to stop the ASP.NET service when the background service crashes? To do that, you’ll need to handle exceptions in the background service and call IHostApplicationLifetime.StopApplication(), like … Read more
The options pattern is an indirect way to dependency inject settings into a registered service. If you’re using code that implements the options pattern, then you’re required to supply an IOptions<T> object. For example, let’s say you’re using the MovieService class and it has the following constructor: This requires you to supply the IOptions<MovieSettings> parameter. … Read more
When you try to call services.BuildServiceProvider(), you’ll get the following warning: Warning ASP0000 Calling ‘BuildServiceProvider’ from application code results in an additional copy of singleton services being created. Consider alternatives such as dependency injecting services as parameters to ‘Configure’. There are two scenarios where you may be calling BuildServiceProvider() because you want to resolve services … Read more
Let’s say you have a background service called DatabaseLoggerService. It runs as a hosted background service and logs messages to the database. It has the following definition: You want your controllers to use this for logging. They don’t need to know about the concrete DatabaseLoggerService class, and they don’t need to know they are actually … Read more
When you want to use NLog in ASP.NET, the first step is to install and configure NLog. Then you can either use NLog directly or fully integrate NLog and ASP.NET. Use NLog directly if you prefer to have static ILogger properties, instead of using dependency injection. The downside of this approach is that you’ll have … Read more
System.Text.Json is the default JSON serializer in ASP.NET Core. It uses the following default serialization settings: To change the settings at the service level for all controllers, call AddJsonOptions() in Startup.ConfigureServices() like this: Note: This example is passing in JsonStringEnumConverter, which makes JsonSerializer use the enum name instead of enum value. When you change the … Read more
The key to unit testing an ApiController class is to mock out all of its dependencies, including the controller’s HttpContext property, like this: If the controller method you’re testing uses anything from the HttpContext, then you’ll want to swap in your own value. Otherwise HttpContext will be null and you’ll get a NullReferenceException. Fortunately Microsoft … Read more
Testing code with a dependency on an event can be tricky. You may be tempted to directly unit test the event handling method. However, that doesn’t prove that code under test is listening for the event. It’s better to really fire the event, and then test side effects of the event handling method. Firing the … Read more
In ASP.NET, when you need a long-running background task, or need to run a task periodically, you can implement IHostedService or BackgroundService and register your class as a hosted service in Startup. As you may have already encountered, if you try to run a long-running background task, the web server will randomly kill your task … Read more
Let’s say you have the following async method you want to test: Here’s how to unit test this: This is awaiting the method you’re testing. To await it, you must make the unit test method return async Task. This example is a bit simplistic. In the real world when you are working with async methods, … Read more
The need to mock static methods in order to add a unit test is a very common problem. It’s often the case that these static methods are in third-party libraries. There are many utility libraries that are completely made up of static methods. While this makes them very easy to use, it makes them really … Read more
