Fix: A possible object cycle was detected
System.Text.Json refuses to serialize graphs with back-references. Set ReferenceHandler.IgnoreCycles, project to a DTO, or mark the back-pointer with [JsonIgnore]. Preserve is a last resort.
Tag: ef-core
68 posts · Page 6 of 7
Fix: SqlException: Timeout expired during EF Core migrations
Migrations use the design-time DbContext, not your runtime CommandTimeout. Set the timeout via UseSqlServer(o => o.CommandTimeout(...)), the connection string Command Timeout, or Database.SetCommandTimeout before Migrate().
Fix: dotnet ef migrations add fails with 'Unable to create an object of type DbContext'
EF Core's design-time tools could not instantiate your DbContext. Either expose a host via WebApplication.CreateBuilder, point to the right startup project, or implement IDesignTimeDbContextFactory.
Fix: The instance of entity type cannot be tracked because another instance with the same key value is already being tracked
EF Core 11 throws when two objects share a primary key inside one DbContext. Detach the old one or update it in place. AsNoTracking on the read prevents the collision.
Fix: A second operation was started on this context instance before a previous operation completed
EF Core throws when two awaits run in parallel on the same DbContext. Await each call sequentially, or get a new DbContext per concurrent unit of work via IDbContextFactory.
Fix: System.InvalidOperationException: No connection string named 'DefaultConnection' could be found
If GetConnectionString returns null in .NET 11, your appsettings.json is missing the key, not copied to the build output, or the wrong environment file is being selected. Three checks fix 95% of cases.
How to Expose an EF Core Database to an AI Agent via MCP
Wire an EF Core 10 DbContext into a Model Context Protocol server so Claude Code, Cursor, or any compliant client can run safe, scoped queries against your database. Covers IDbContextFactory lifetime, read-only projections, schema discovery tools, AsNoTracking, parameterised filters, row-level scoping, and the destructive-tool gates you need before letting an agent touch UPDATE.
How to detect N+1 queries in EF Core 11
A practical guide to spotting N+1 queries in EF Core 11: what the pattern looks like in real code, how to surface it via logging, diagnostic interceptors, OpenTelemetry, and a test that fails the build when a hot path regresses.
How to use compiled queries with EF Core for hot paths
A practical guide to EF Core 11 compiled queries: when EF.CompileAsyncQuery actually wins, the static-field pattern, the Include and tracking gotchas, and how to benchmark before and after so you can prove it was worth the extra ceremony.
How to write integration tests against a real SQL Server with Testcontainers
A complete guide to running ASP.NET Core integration tests against a real SQL Server 2022 using Testcontainers 4.11 and EF Core 11: WebApplicationFactory wiring, IAsyncLifetime, swapping the DbContext registration, applying migrations, parallelism, Ryuk cleanup, and CI gotchas.