19 posts tagged with "LINQ"

With EF Core, you read rows by writing a LINQ query in C#, and EF Core turns it into SQL. The part that surprises people is timing: building the query sends nothing, and only ToListAsync or a similar call goes to MySQL. This guide reads rows with Where and OrderBy, shows the SQL each piece becomes, and proves the timing with real logs.

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.

Entity Framework Core maps each database table to an entity class and gives you a DbContext to query them. Most of the time you read from a single DbSet, but sooner or later you need data that lives across two tables — exactly what a SQL JOIN gives you. In EF Core, the lambda-based Join() method does the same job, and it translates into real SQL that runs on the database server.

Everything below targets .NET 10 and EF Core 10, the current Long Term Support releases.