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In this article, let's learn about how to unit test Middlewares in WebAPI in ASP.NET Core.

Table of Contents

1. Introduction
2. Why Unit Testing?
3. What is a Middleware?
4. When to use Middleware?
5. Unit Testing Middleware
6. Summary

Introduction

Unit testing is a crucial practice in software development that ensures the reliability and functionality of code components. When it comes to ASP.NET Core middleware, unit testing becomes essential for custom middleware that includes custom behavior. In this article, we will explore the importance of unit testing middleware, understand what middleware is, and provide practical examples using C# to demonstrate how to effectively test ASP.NET Core middleware.

Why Unit Testing ?

Unit testing custom middleware allows developers to verify the behavior and functionality of their code. It provides confidence that the middleware functions as intended, adding value to the application. Unit tests catch bugs early in the development process, allowing for quick identification and resolution. By testing middleware, developers can isolate and validate specific behaviors without the need for complex integration testing, reducing the dependencies on external components.

Note: If you have not done so already, I recommend you read the article on Implementing TDD in C# .Net.

What is a Middleware ?

In the context of ASP.NET Core, middleware refers to components that are placed in the application's request pipeline. It sits between the client and the server, processing requests and generating responses. Middleware performs various tasks, such as authentication, logging, exception handling, and modifying HTTP headers. Each middleware component is responsible for a specific functionality, and they are executed sequentially as the request flows through the pipeline.

Note: If you have not done so already, I recommend you read the article on Introducing Middleware in ASP.NET.

When to use Middleware ?

Custom middleware is often required when the built-in ASP.NET Core middleware does not meet specific application requirements. It allows developers to extend the request pipeline with their own logic. Some scenarios where custom middleware might be used include:

• Adding security headers to HTTP responses.
• Implementing custom logging or error handling.
• Modifying request or response data.
• Enforcing specific authorization rules.

Unit Testing Middleware

Let's consider an example of unit testing an ASP.NET Core middleware named AddingSecurityHeadersMiddleware. This middleware adds security headers to the HTTP response.

To begin, we create a test class called AddSecurityHeadersMiddlewareTests and write a test method named InvokeAsync_SetsExpectedResponseHeaders. In this test, we want to ensure that the expected response headers are added by the middleware.

Now let's add test to this middleware. The steps are,

In the test method, we set up the dependencies required for the test. This includes creating an instance of the HttpContext using DefaultHttpContext and mocking the RequestDelegate, which represents the next middleware in the pipeline. To isolate the test, we use an action that returns a completed task instead of invoking the actual next middleware.

Next, we instantiate the middleware by passing the mocked RequestDelegate to its constructor. Then, we call the InvokeAsync method of the middleware, passing the created HttpContext.

Finally, we assert the values of the response headers added by the middleware by accessing the Response.Headers collection of the HttpContext. We compare the obtained values with the expected values using assertions.

Summary

Unit testing custom ASP.NET Core middleware ensures the reliability and correctness of the code. By testing middleware components, developers can verify their behavior and functionality in isolation, reducing the need for complex integration tests. Through practical examples and the use of mocking techniques, developers can effectively test their middleware, enhancing the overall quality of ASP.NET Core applications.