C# 15 Collection Expression Arguments: Enterprise Adoption Considerations

The C# language continues its rapid evolution with the introduction of Collection Expression Arguments in C# 15. This preview feature, announced alongside .NET 11 Preview 1 in February 2026, represents a significant refinement to the collection initialization syntax that landed in C# 12. For enterprise teams evaluating their adoption roadmap, understanding the practical implications of this feature requires looking beyond the syntax convenience to examine team-wide impacts on maintainability, code review processes, and long-term codebase health.

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What Collection Expression Arguments Actually Change

Prior to C# 15, collection expressions provided a cleaner syntax for initializing collections using square bracket notation instead of traditional constructor calls. C# 15 extends this with the ability to pass constructor arguments directly within the expression using a with(...) element as the first element.

This addresses a common performance concern when pre-allocating collections with known sizes. Rather than calling the constructor separately and then populating the collection, teams can now do both in a single expression.

Enterprise Implications: When Convenience Becomes Technical Debt

The adoption question for enterprise teams isn't whether the feature works—it's whether the team will abuse it. Here's the honest assessment:

Arguments For Adoption

The initial capacity scenario is legitimate. When building large in-memory collections, specifying capacity upfront eliminates resize overhead. In high-throughput API scenarios or data processing pipelines, this matters. The feature makes the optimization explicit at the declaration point rather than requiring a separate constructor call.

Additionally, custom collection types with factory methods gain a more uniform initialization pattern. Teams using custom collection implementations for domain-specific behavior can now initialize them with the same syntax as built-in types.

Arguments Against Immediate Adoption

The risk lies in over-application. Junior developers seeing this syntax for the first time may reach for it everywhere, including scenarios where simple array or list initialization is clearer. The with(...) syntax adds cognitive load during code review, particularly when the argument choices aren't immediately obvious.

There's also the compatibility consideration: this is a preview feature in .NET 11 Preview 1. Enterprise teams typically prefer waiting for LTS alignment before adopting language features in production code. C# 15 will ship with .NET 11 in November 2026, meaning production readiness likely lands in Q1 2027.

Governance Framework for Enterprise Teams

Teams considering adoption should establish clear guidelines:

Approve for performance-critical paths: Where profiling identifies collection allocation as a bottleneck, permit the syntax with documented justification in code comments.

Default to simplicity elsewhere: For typical application code, the marginal performance gain rarely justifies the syntax complexity. Establish team conventions that default to traditional initialization unless a specific performance requirement exists.

Code review checkpoints: Include syntax appropriateness in review checklists. Questions to ask: Does the argument provide measurable value? Is the choice documented? Could a simpler alternative work equally well?

The Timing Question

For teams on .NET 10 LTS, the realistic adoption timeline is late 2026 or early 2027. This gives ample time to establish governance patterns before the feature lands in production environments. The key is not waiting until the feature ships to have the conversation—start the policy discussions now so teams are prepared when .NET 11 reaches stability.

The practical reality is that most enterprise applications won't see meaningful performance differences from this feature. But teams that establish clear adoption guidelines will avoid the scattered, inconsistent usage patterns that emerge when developers adopt new syntax without team-level coordination.

Frequently Asked Questions

Is C# 15 collection expression arguments available in production now?

No. Collection expression arguments are currently in preview with .NET 11 Preview 1. The feature will ship with .NET 11 in November 2026 and should be considered production-ready in early 2027 after sufficient community validation.

What's the performance benefit of specifying initial capacity?

When you know the approximate size upfront, specifying initial capacity prevents the collection from repeatedly allocating larger internal buffers as elements are added. For collections expected to hold thousands of items, this can reduce memory allocations significantly.

Should we ban this feature in our codebase?

Not necessarily. A ban creates friction when legitimate use cases arise. Instead, establish guidance for when it's appropriate—performance-critical code with measured requirements—and default to simpler alternatives elsewhere.

How does this interact with custom collection types?

The with syntax can pass arguments to any constructor or factory method the collection type exposes. This makes custom collection types more ergonomic to initialize but requires careful API design to avoid exposing internal implementation details.

What's the learning curve for teams upgrading from C# 12?

Minimal for developers already familiar with collection expressions. The new with syntax is intuitive. The challenge is establishing team discipline about when to use it, not understanding how it works.

Can this feature be used with immutable collections?

Yes, provided the collection type exposes appropriate constructors or factory methods. The syntax works with any collection type that supports the underlying construction pattern, though immutable collections may have different performance characteristics than mutable ones.