Use a different testing framework

Oct 13, 2025 by pinkandtrash.com 18 views

Cannot Combine WebApplicationFactory with Customized Dependency Injection via ClassConstructor

Problem

When using the WebApplicationFactory in ASP.NET Core to test an application, it can be challenging to combine it with customized dependency injection via ClassConstructor. This issue arises when trying to inject dependencies, such as data factories, into test classes to easily create data. The problem is that the ClassDataSource only allows parameterless constructors, and as soon as parameters (in this case, dependencies) are added, the build fails with a "must be a non-abstract type with a public parameterless constructor" error.

Reproduction

To reproduce this issue, let's consider an example where we have a TestFactory class that depends on an AppDbContext. The AppDbContext is used and registered throughout the API that is being tested.

public class TestFactory
{
    private readonly AppDbContext _dbContext;
    
    public TestFactory(AppDbContext dbContext)
    {
        _dbContext = dbContext;
    }
    
    // Methods that use the DbContext
}

As we can see, the TestFactory class has a parameterized constructor that takes an AppDbContext instance. However, when we try to use the ClassDataSource to create an instance of the TestFactory class, it fails because the ClassDataSource only allows parameterless constructors.

Solution

To solve this issue, we can use ClassConstructor helpers to allow creation of classes with parameters. However, when we do this, the test fails at runtime because the ServiceCollection from the DiClassConstructor and the ServiceCollection from the regular API are not the same. The TestFactory receives a DbContext, but it's not the same DbContext that is used by the WebApplicationFactory, so changes made through one aren't visible to the other.

Revised Code

Let's take a look at the revised code that demonstrates how to use ClassConstructor helpers to create classes with parameters.

// MinimalApi/Program.cs
using Microsoft.EntityFrameworkCore;
using MinimalApi.Data;

var builder = WebApplication.CreateBuilder(args);

// Add DbContext
builder.Services.AddDbContext<AppDbContext>(options =>
    options.UseInMemoryDatabase("TestDb"));

var app = builder.Build();

app.MapGet("/", () => "Hello World!");

app.Run();

// Enable testing
public partial class Program {}

// MinimalApi/Data/AppDbContext.cs
using Microsoft.EntityFrameworkCore;

namespace MinimalApi.Data;

public class AppDbContext : DbContext
{
    public AppDbContext(DbContextOptions<AppDbContext> options) : base(options) { }
    
    public DbSet<User> Users => Set<User>();
}

public class User
{
    public int Id { get; set; }
    public string Name { get; set; } = string.Empty;
}

// TestProject/DataServices/TestFactory.cs
using MinimalApi.Data;

namespace TestProject.DataServices;

public class TestFactory
{
    private readonly AppDbContext _dbContext;
    
    public TestFactory(AppDbContext dbContext)
    {
        _dbContext = dbContext;
    }
    
    public async Task CreateUserAsync(string name)
    {
        _dbContext.Users.Add(new User { Name = name });
        await _dbContext.SaveChangesAsync();
    }
    
    public async Task<IEnumerable<User>> GetAllUsersAsync()
    {
        return await _dbContext.Users.ToListAsync();
    }
}

// TestProject/AppFactory.cs
using Microsoft.AspNetCore.Mvc.Testing;
using Microsoft.EntityFrameworkCore;
using MinimalApi;
using MinimalApi.Data;
using TUnit.Core.Interfaces;

namespace TestProject;

public class AppFactory : IAsyncInitializer, IAsyncDisposable
{
    public WebApplicationFactory<Program>? Factory { get; private set; }

    public async Task InitializeAsync()
    {
        Factory = new WebApplicationFactory<Program>().WithWebHostBuilder(builder =>
        {
            builder.ConfigureServices(services =>
            {
                // test config
            });
        });
        
        await Task.CompletedTask;
    }

    public async ValueTask DisposeAsync()
    {
        if (Factory != null)
        {
            await Factory.DisposeAsync();
        }
    }
}

// TestProject/DiClassConstructor.cs
using Microsoft.Extensions.DependencyInjection;
using TestProject.DataServices;
using TUnit.Core;
using TUnit.Core.Interfaces;

namespace TestProject;

public class DiClassConstructor : IClassConstructor, ITestEndEventReceiver
{
    private static readonly IServiceProvider _serviceProvider;
    private AsyncServiceScope _scope;

    static DiClassConstructor()
    {
        var services = new ServiceCollection();
        
        // Register TestFactory which depends on AppDbContext
        services.AddTransient<TestFactory>();
        
        _serviceProvider = services.BuildServiceProvider();
    }

    public T Create<T>(ClassConstructorMetadata classConstructorMetadata) where T : class
    {
        _scope = _serviceProvider.CreateAsyncScope();
        return ActivatorUtilities.CreateInstance<T>(_scope.ServiceProvider);
    }

    public ValueTask OnTestEnd(TestContext testContext)
    {
        return _scope.DisposeAsync();
    }
}

// TestProject/UserTests.cs
using Microsoft.AspNetCore.Mvc.Testing;
using MinimalApi;
using TestProject.DataServices;
using TUnit.Core;

namespace TestProject;

[ClassConstructor<DiClassConstructor>]
public class UserTests
{
    private readonly TestFactory _testFactory;
    
    public UserTests(TestFactory testFactory)
    {
        _testFactory = testFactory;
    }
    
    [Test]
    [ClassDataSource<AppFactory>(Shared = SharedType.PerTestSession)]
    public async Task CreateUser_ShouldAddUserToDatabase(AppFactory appFactory)
    {
        // This test will fail because TestFactory has no valid AppDbContext
        // The AppDbContext is from the AppFactory but DiClassConstructor doesn't know about it
        await _testFactory.CreateUserAsync("Test User");
        
        // We'd need to get a new TestFactory instance with the correct DbContext
        // But there's no clean way to do this with the current TUnit architecture
        var users = await _testFactory.GetAllUsersAsync();
        
        // This assertion will fail because TestFactory has a different AppDbContext
        Assert.AreEqual(1, users.Count());
    }
}

Conclusion

In conclusion, combining WebApplicationFactory with customized dependency injection via ClassConstructor can be challenging. However, by using ClassConstructor helpers to create classes with parameters, we can overcome the issue of parameterless constructors. Nevertheless, the test will still fail at runtime because the ServiceCollection from the DiClassConstructor and the ServiceCollection from the regular API are not the same. To resolve this issue, we need to find a way to share the same ServiceCollection between the DiClassConstructor and the regular API.

Future Work

To resolve this issue, we can explore the following options:

Use a shared ServiceCollection: We can create a shared ServiceCollection that is used by both the DiClassConstructor and the regular API. This way, the same ServiceCollection is used to create instances of classes with parameters.
Use a custom ClassConstructor: We can create a custom ClassConstructor that takes into account the ServiceCollection from the regular API. This way, the custom ClassConstructor can create instances of classes with parameters using the correct ServiceCollection.
Use a different testing framework: We can explore using a different testing framework that supports customized dependency injection out of the box.

By exploring these options, we can find a solution that works for our specific use case and allows us to combine WebApplicationFactory with customized dependency injection via ClassConstructor.
Q&A: Cannot Combine WebApplicationFactory with Customized Dependency Injection via ClassConstructor

Q: What is the issue with combining WebApplicationFactory with customized dependency injection via ClassConstructor?

A: The issue arises when trying to inject dependencies, such as data factories, into test classes to easily create data. The problem is that the ClassDataSource only allows parameterless constructors, and as soon as parameters (in this case, dependencies) are added, the build fails with a "must be a non-abstract type with a public parameterless constructor" error.

Q: How can I solve this issue?

A: To solve this issue, you can use ClassConstructor helpers to allow creation of classes with parameters. However, when you do this, the test fails at runtime because the ServiceCollection from the DiClassConstructor and the ServiceCollection from the regular API are not the same.

Q: What is the cause of the runtime failure?

A: The cause of the runtime failure is that the TestFactory receives a DbContext, but it's not the same DbContext that is used by the WebApplicationFactory, so changes made through one aren't visible to the other.

Q: How can I resolve this issue?

A: To resolve this issue, you can explore the following options:

Use a shared ServiceCollection: You can create a shared ServiceCollection that is used by both the DiClassConstructor and the regular API. This way, the same ServiceCollection is used to create instances of classes with parameters.
Use a custom ClassConstructor: You can create a custom ClassConstructor that takes into account the ServiceCollection from the regular API. This way, the custom ClassConstructor can create instances of classes with parameters using the correct ServiceCollection.
Use a different testing framework: You can explore using a different testing framework that supports customized dependency injection out of the box.

Q: What are the benefits of using a shared ServiceCollection?

A: The benefits of using a shared ServiceCollection include:

Consistency: Using a shared ServiceCollection ensures that the same dependencies are used throughout the application.
Reusability: A shared ServiceCollection can be reused across multiple tests and applications.
Simplified testing: Using a shared ServiceCollection simplifies testing by eliminating the need to create and manage multiple ServiceCollections.

Q: What are the benefits of using a custom ClassConstructor?

A: The benefits of using a custom ClassConstructor include:

Flexibility: A custom ClassConstructor can be tailored to meet the specific needs of your application.
Customization: A custom ClassConstructor can be used to create instances of classes with parameters using the correct ServiceCollection.
Improved performance: A custom ClassConstructor can improve performance by reducing the overhead of creating and managing multiple ServiceCollections.

Q: What are the benefits of using a different testing framework?

A: The benefits of using a different testing framework include:

Improved support for customized dependency injection: Some testing frameworks provide better support for customized dependency injection out of the box.
Simplified testing: Using a different testing framework can simplify testing by eliminating the need to create and manage multiple ServiceCollections.
Improved performance: Using a different testing framework can improve performance by reducing the overhead of creating and managing multiple ServiceCollections.

Q: What are some popular testing frameworks that support customized dependency injection?

A: Some popular testing frameworks that support customized dependency injection include:

xUnit: xUnit is a popular testing framework that provides good support for customized dependency injection.
NUnit: NUnit is another popular testing framework that provides good support for customized dependency injection.
MSTest: MSTest is a testing framework that provides good support for customized dependency injection.

Q: How can I choose the right testing framework for my application?

A: To choose the right testing framework for your application, consider the following factors:

Support for customized dependency injection: Choose a testing framework that provides good support for customized dependency injection.
Simplicity: Choose a testing framework that is easy to use and understand.
Performance: Choose a testing framework that provides good performance.
Community support: Choose a testing framework that has a large and active community.