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Tag: linq

11 posts · Page 1 of 2

2026-09-29 errors efcore dotnet

Fix: EF Core `Contains` on a static readonly `IList<T>` or `ISet<T>` could not be translated

EF Core 8, 9 and 10 fail to translate Contains when the list is a static readonly field typed as IList, ICollection, ISet or IReadOnlySet. Call Enumerable.Contains explicitly, or upgrade to EF Core 11.

2026-09-14 errors efcore dotnet

Fix: InvalidOperationException: Nullable object must have a value in an EF Core 11 projection

EF Core throws this when a Select reads a SQL NULL into a non-nullable int, decimal or DateTime. Cast the member to its nullable type and add ?? default, or guard the navigation with a null check.

2026-09-12 ef-core ef-core-11 dotnet-11

How to escape % and _ wildcards in EF.Functions.Like and StartsWith queries in EF Core 11

StartsWith, EndsWith and Contains already escape % and _ for you in EF Core 11, but EF.Functions.Like does not. Here is the SQL EF generates, a reusable escape helper, and the escapeCharacter overload that makes it work on SQL Server, SQLite and PostgreSQL.

2026-09-12 comparison csharp dotnet

Lookup<TKey, TElement> vs Dictionary<TKey, List<TValue>> for duplicate keys in C#

Use ToLookup when you group once and only read: it is immutable, returns an empty sequence for missing keys, accepts null keys and keeps first-seen order. Use Dictionary<TKey, List<TValue>> when the groups change after construction or cross a JSON boundary.

2026-08-23 csharp linq dotnet

How to tell whether an IEnumerable<T> has already been materialized in C#

There is no HasBeenEnumerated flag on IEnumerable<T>. Here is what TryGetNonEnumeratedCount actually checks, why Enumerable.Range passes an ICollection<T> test, and the guard that avoids a wasted ToList().

2026-08-23 ef-core linq csharp

How to write reusable LINQ predicates that EF Core can translate in Where, Select, and OrderBy

A bool helper method throws "could not be translated". An Expression<Func<T, bool>> does not. Here is how to compose, nest, and reuse expression trees in EF Core 11 without LINQKit, with the real SQL for every case.

2026-08-19 errors ef-core dotnet

Fix: "An exception was thrown while attempting to evaluate a LINQ query parameter expression" in EF Core 11

EF Core throws this when a client-evaluated piece of your query throws while EF evaluates it. Read InnerException, turn on EnableSensitiveDataLogging, and move the null check outside the lambda.

2026-07-21 errors csharp dotnet

Fix: System.InvalidOperationException: Sequence contains no elements

This exception means you called .First() or .Single() on an empty sequence. Use FirstOrDefault/SingleOrDefault and null-check, or guard the query, or fix why the source is empty.

2026-06-12 dotnet-11 csharp linq

LINQ Gets FullJoin and Selector-Free Joins in .NET 11 Preview 5

.NET 11 Preview 5 adds a brand-new FullJoin operator to LINQ plus tuple-returning overloads for Join, LeftJoin, RightJoin, and GroupJoin that drop the result selector entirely.

2026-05-21 comparison csharp linq

IEnumerable vs IAsyncEnumerable vs IQueryable in C#: which one should the method return?

Three sequence interfaces, three execution models. Use IQueryable when a database can translate the query, IAsyncEnumerable when the producer is async and you want to stream, IEnumerable for everything else in memory.