Fix: The JSON value could not be converted to System.DateTime
System.Text.Json only accepts ISO 8601 strings for DateTime. Send 2026-05-08T14:00:00Z or register a JsonConverter that parses your format. Empty strings and Unix timestamps both throw.
Tag: errors
104 posts · Page 11 of 11
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.