Overriding services in ASP.NET Core integration tests

11 Aug 2023 ASP.NET Testing

Let's say you have the following ASP.NET Core project.

namespace PlaygroundApi
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var builder = WebApplication.CreateBuilder(args);
            builder.Services.AddScoped<IPaymentService>(x => new StripePaymentService());

            var app = builder.Build();
            app.MapGet("/", (IConfiguration config) =>
            {
                var projectKey = config.GetValue<string>("ProjectKey");
                return $"ProjectKey is: '{projectKey}'";
            });
            app.Run();
        }
    }
}

It has a IPaymentService scoped service injected with a StripePaymentService implementation.

For the needs of integration testing we want to mock the payment service, and not use the real StripePaymentService implementation. To do that, we can use the ConfigureTestServices method extension in the TestServer to override the IPaymentService implementation with a MockPaymentService.

using Microsoft.AspNetCore.Mvc.Testing;
using Microsoft.AspNetCore.TestHost;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;

namespace PlaygroundApi.Tests
{
    public class UnitTest1
    {
        private readonly WebApplicationFactory<PlaygroundApi.Program> _webAppFactory;

        public UnitTest1()
        {
            _webAppFactory = new WebApplicationFactory<PlaygroundApi.Program>()
                .WithWebHostBuilder(builder =>
                {
                    builder.ConfigureTestServices(services =>
                    {
                        services.AddScoped<IPaymentService>(x => new MockPaymentService());
                    });
                });
        }

        [Fact]
        public void Test_payment()
        {
            using var scope = _webAppFactory.Services.CreateScope();
            var paymentService = scope.ServiceProvider.GetRequiredService<IPaymentService>();

            // Test proof that the payment service type now is the mock.
            Assert.Equal(typeof(MockPaymentService), paymentService.GetType());
        }
    }
}