.NET 10 LINQ and Performance

.NET 10 LINQ and Performance Updates: Insights from a Senior Developer

LINQ’s IEnumerable, Where, LeftJoin

I work with .NET daily and test new releases to see what’s useful. The .NET 10 preview caught my attention — not for big announcements, but for small, practical improvements. These changes target issues senior developers face in production code. Here’s what I found in .NET 10, with examples, performance notes, and why it matters.

𝟭. LINQ’s IEnumerable

One improvement is in LINQ’s IEnumerable. In .NET 9, the abstraction penalty was high — benchmarks showed 83% overhead in some cases, for an API handling large datasets, that hurts. In .NET 10, it’s down to 10%. I tested this on a data pipeline processing customer records. With the preview runtime, throughput increased by 15%. The code didn’t change:

var filtered = customers.Where(c => c.IsActive).Select(c => c.Name);
foreach (var name in filtered) { Process(name); }

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𝟮. Where

In .NET 9, Where caused allocations that slowed it down. In .NET 10, it’s more efficient — fewer heap hits, less garbage collection.

▪️Pros: better speed for CPU-bound tasks.

▪️Cons: no impact if your bottleneck is I/O or queries.

🔹Best practice: profile your code to confirm the gain.

𝟯. LeftJoin

Another feature is LeftJoin, a new LINQ operator. I’ve written complex left joins before using GroupJoin and SelectMany. For an API matching orders to optional discounts, it looked like this:

var result = orders.GroupJoin(discounts, 
    o => o.Id, 
    d => d.OrderId, 
    (o, ds) => new { Order = o, Discounts = ds.DefaultIfEmpty() })
    .SelectMany(x => x.Discounts.Select(d => new { x.Order, Discount = d }));

With .NET 10’s LeftJoin, it’s simpler: