272 posts tagged with "Issues"

The build stopped with error CS1929: 'DbSet<Category>' does not contain a definition for 'FirstOrDefaultAsync' and the best extension method overload 'AsyncEnumerable.FirstOrDefaultAsync…' requires a receiver of type…. ToListAsync failed with a different message, CS0411. On .NET 10, both mean the same simple thing: the file is missing using Microsoft.EntityFrameworkCore;.

FirstOrDefaultAsync() returned a row, and EF Core logged a warning with it: The query uses the 'First'/'FirstOrDefault' operator without 'OrderBy' and filter operators. This may lead to unpredictable results. The word "first" needs an order to mean anything, and SQL doesn't give you one by itself. An OrderBy or a Where makes the warning go away, and the answer reliable.

My paging endpoint answered 200 OK, and the log had a warning: The query uses a row limiting operator ('Skip'/'Take') without an 'OrderBy' operator. This may lead to unpredictable results. It's right to warn. Without ORDER BY, MySQL returned the rows in the order of whichever index it happened to read, so the next query could give a different page. Adding a fully unique sort fixes it.

I added StringComparison.OrdinalIgnoreCase to a Contains in an EF Core query, to make sure the search ignored case. With Oracle's MySQL provider, it failed with an unusual message: Expression '[SqlExpressions.SqlFunctionExpression] COLLATE utf8mb4_bin' in the SQL tree does not have a type mapping assigned. The search was already ignoring case, because the column's collation does. Removing the argument fixed it.

A search for usb with Contains found five products named USB-C …. The same search with StartsWith found none, and neither threw an error. With Oracle's MySQL provider for EF Core, StartsWith and EndsWith compare with a case-sensitive collation, while Contains uses the column's own, which ignores case. EF.Functions.Like gives you a prefix search that follows the column.

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.