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:

var result = orders.LeftJoin(discounts, 
    o => o.Id, 
    d => d.OrderId, 
    (o, d) => new { Order = o, Discount = d });

Testing 10,000 orders with sparse discounts, response time dropped from 150ms to 130ms.

▪️Pros: cleaner code, small performance boost.

◾Cons: it’s preview-only, so it may change.

🔹Best practice: use it for readability, but monitor updates.

📝End Note

These updates show Microsoft addressing real problems. The IEnumerable fix reduces overhead without extra work. **LeftJoin **simplifies common tasks. I’ve seen frameworks overpromise before, but these feel targeted — built for developers who need efficiency, not flash.

There’s more in C# 14, like pattern matching updates, that I’m still testing. For now, .NET 10’s value is in these quiet fixes. I’m reviewing a batch processor I wrote last year — LeftJoin could streamline its data merges, and the IEnumerable tweak might cut memory use. These aren’t huge shifts, but they improve what’s already there.

Try the .NET 10 preview on your own code. Run a LINQ-heavy section and measure the difference.

It’s not about rewriting — it’s about better results with less effort. That’s what I see in this release.

Rely on Coffeesss…

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