.NET Framework and Core for Application Development
View All Tags
SingleOrDefaultAsync threw InvalidOperationException: Sequence contains more than one element. The "OrDefault" in the name only covers zero rows. Single means "there's at most one", and when MySQL returns two it treats that as a bug and says so. Usually it's right: the filter is missing or too loose.
A request for a category that doesn't exist answered 500, with InvalidOperationException: Sequence contains no elements. The database was fine, and so was the SQL: it ran and found nothing. FirstAsync treats "nothing" as an exception. For an ID a client sent, nothing is a normal answer, so the code should use FirstOrDefaultAsync and reply 404.
An endpoint started some work in the background and answered 200 OK straight away. Half a second later the log said ObjectDisposedException: Cannot access a disposed context instance. The DbContext belongs to the request: when the request ended, ASP.NET Core disposed of it, and the background task was still holding it. Background work needs a context of its own.
To save time, I started three EF Core queries together and waited for them with Task.WhenAll. The API answered 500: A second operation was started on this context instance before a previous operation completed. Then I ran the same code again, and it worked. A DbContext can only do one thing at a time, and when you break that rule, whether you notice depends on timing.
I loaded a row to check it existed, built a new object with the same ID and the changed values, and called db.Categories.Update. EF Core refused: The instance of entity type 'Category' cannot be tracked because another instance with the same key value for {'Id'} is already being tracked. A context allows one object per row. The simple fix is to change the object you loaded, with no Update call at all.
A POST with an empty JSON body answered 400 Bad Request with no explanation, and the log said JSON deserialization for type 'Dewiride.Inventory.Data.Entities.Category' was missing required properties including: 'name'. The C# required keyword on my entity was doing its job, too early and too quietly. A request record with validation attributes says the same thing in a way a client can read.
An endpoint that returned EF Core entities answered 500, with JsonException: A possible object cycle was detected. The path in the message gave it away: $.Products.Category.Products.Category.Products…. A category has products, and each product points back to its category, so the JSON would never end. The fix isn't a JSON setting: it's not returning entities in the first place.
The database operation was expected to affect 1 row(s), but actually affected 0 row(s); data may have been modified or deleted since entities were loaded. EF Core expected its UPDATE to change one row, and MySQL said none. I got this message three different ways while building an API, and each needs a different answer. Telling them apart takes one look at the SQL.
Every request to my minimal API answered 500, and the log said Failure to infer one or more parameters. The message includes a small table, and one row in it had the answer: clock | UNKNOWN. My endpoint asked for a TimeProvider, and nothing had registered one. One line in Program.cs fixed it.
The API itself worked, but /openapi/v1.json started answering 500: InvalidCastException: Unable to cast object of type 'System.Attribute[]' to type 'System.Collections.Generic.IEnumerable1[System.ComponentModel.DataAnnotations.ValidationAttribute]'.The cause was a record used with[AsParameters], with validation attributes on its constructor parameters. It's a bug in ASP.NET Core 10, and adding property:` to each attribute avoids it.
