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2026-11-08 dotnet csharp linq

How to do DistinctBy but keep the last element of each duplicate in C#, in source order

Enumerable.DistinctBy keeps the first element per key. To keep the last one and still return results in the order they appear in the source, buffer the input, scan it backwards with a HashSet, and yield the survivors forwards. Eight implementations checked for order, null keys, double enumeration and argument validation on .NET 10.0.10 and .NET 11 RC3, and benchmarked on .NET 10.

2026-11-08 dotnet csharp linq

Why LINQ ThenBy computes every secondary key, and how to compute them only on ties

OrderBy(...).ThenBy(...) runs every key selector for every element before it compares anything, even when the OrderBy keys are already unique. Measured on .NET 10 and .NET 11 RC3: what the runtime does, why the popular lazy-key workaround still computes keys it never needs, and three patterns that call an expensive secondary key only for tied elements.

2026-11-06 dotnet csharp linq

Is List<T>.Sort stable in C#? Why equal elements change order

List<T>.Sort, Array.Sort, Span<T>.Sort and ImmutableArray.Sort are unstable introsorts, so items with equal keys can swap places. Enumerable.OrderBy is stable. Measured on .NET 10: why lists of 16 or fewer items look stable, why three items are not, how to get a stable in-place sort, and what it costs.

2026-10-31 comparison csharp dotnet

yield return iterator vs returning a List<T>: what an IEnumerable method actually costs in C#

A yield return method costs one 40-byte state machine object per call no matter how many items it produces; a List<T> costs the whole backing array up front. Measured on .NET 10.0.10: iterator wins on memory and early exit, List<T> wins when the caller enumerates more than once, needs Count, or calls ToList().

2026-10-27 csharp dotnet performance

How to iterate over two collections in lockstep without heap allocations in C#

Enumerable.Zip allocates 144-208 bytes per call and runs about 10x slower than a plain loop. For arrays and lists, a for loop over two spans sliced to the same length is allocation-free and the fastest option; for foreach syntax, a small ref struct zip enumerator gets within 25% of it. Measured on .NET 10.0.10 and .NET 11 RC 3.

2026-09-29 errors ef-core 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 ef-core 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().