What problem does it solve? Unit tests verify logic in isolation but miss bugs that only appear when components interact — middleware ordering, EF Core query translation, HTTP serialization, Blazor rendering, and Avalonia binding failures. This Skill provides concrete patterns for testing .NET systems at their real integration boundaries so those bugs are caught before release. ## Core Features & Use Cases - HTTP API testing: Boots your real Program.cs in-process with WebApplicationFactory<T> and asserts on response status, headers, and serialized bodies. - Database testing: Runs EF Core queries against real PostgreSQL, SQL Server, or Redis instances via Testcontainers, with transaction-rollback isolation between tests. - Browser and desktop UI testing: Drives Blazor, Razor Pages, and MVC views in real browsers with Microsoft.Playwright, and tests Avalonia views headlessly with Avalonia.Headless.XUnit. - Use Case: You changed an EF Core query and an API endpoint. Write a Testcontainers test that runs the query against real Postgres, plus a WebApplicationFactory test asserting the endpoint returns 201 with the correct DTO — catching SQL translation and serialization bugs unit tests would miss. ## Quick Start Write an integration test for my ASP.NET Core task endpoint using WebApplicationFactory and Testcontainers with PostgreSQL.
