.NET app broken in production.Fixed this session.

PM assigned in 10 minutes, .NET engineer begins diagnosis within the hour. ASP.NET Core 500s are most commonly caused by: a missing or misconfigured appsettings.json key accessed via IConfiguration, a DbContext that was registered as Singleton instead of Scoped causing thread-safety violations, an unhandled exception in middleware that bypasses the global exception handler, or a broken dependency injection registration after a NuGet upgrade. The engineer pulls Event Viewer logs or Application Insights traces, checks the structured log output, and traces the exact exception call stack. Most ASP.NET Core 500 incidents resolve within a single 4-hour session.

Yes. EF Core migration failures fall into a few root causes: a migration that was generated against a different database state than the target environment (often caused by a merge conflict in the migrations folder), a pending migration that conflicts with a manually applied schema change, or a foreign key constraint violation during the migration transaction. The engineer runs dotnet ef migrations script to inspect the SQL, checks the __EFMigrationsHistory table on the target database, and either creates a corrective migration or applies a targeted SQL fix to bring the schema and EF model into alignment. The approach is documented so the issue cannot recur.

NuGet dependency conflicts after a major .NET version upgrade are diagnosed using dotnet list package --outdated and the NuGet dependency resolver warnings in the build output. Common failure patterns: two packages depending on incompatible versions of Microsoft.Extensions.DependencyInjection, a transitive dependency pinned to a netstandard2.0 target that does not support net8.0, or a package using internal BCL APIs that were removed in .NET 8. The engineer identifies the conflict graph, updates or replaces the offending package, and adds an explicit version override in the .csproj if needed. A bindingRedirect or global.json pin is added to prevent recurrence in CI.

Memory leaks from undisposed IDisposable objects are diagnosed using dotnet-counters to observe gc-heap-size over time, followed by a memory dump using dotnet-dump or ProcDump analyzed in WinDbg or dotnet-dump analyze. The most common offenders: HttpClient instances created inside loops instead of using IHttpClientFactory, DbContext objects registered as Singleton leaking database connections, or event handler subscriptions holding references to large object graphs preventing GC collection. The engineer identifies the leak source, patches the dispose chain, and verifies with a load test that heap growth stops. A using statement or IDisposable pattern review across the codebase is included.

Every QuickHire .NET engineer passes: a live debugging exercise on a broken ASP.NET Core application with a hidden IDisposable memory leak and an EF Core lazy-loading bug, a written assessment covering C# async/await internals (SynchronizationContext, ConfigureAwait), ASP.NET Core middleware pipeline, Dependency Injection lifetime scopes (Singleton vs Scoped vs Transient), and EF Core query generation, plus a portfolio review of production .NET applications. Only the top 3% pass all stages. PM matches engineers to your stack - .NET version, EF Core vs Dapper, Blazor vs Razor Pages vs minimal APIs, and cloud target (Azure App Service, AKS, AWS Lambda via Lambda Runtime).

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