What's new in C# 15

C# 15 includes the following new features. Try these features by using the latest Visual Studio 2026 insiders version or the .NET 11 preview SDK:

• Collection expression arguments
• Union types

C# 15 is the latest C# preview release. .NET 11 preview versions support C# 15. For more information, see C# language versioning.

You can download the latest .NET 11 preview SDK from the .NET downloads page. You can also download Visual Studio 2026 insiders, which includes the .NET 11 preview SDK.

The "What's new in C#" page adds new features when they're available in public preview releases. The working set section of the roslyn feature status page tracks when upcoming features are merged into the main branch.

You can find any breaking changes introduced in C# 15 in our article on breaking changes.

Note

We're interested in your feedback on these features. If you find issues with any of these new features, create a new issue in the dotnet/roslyn repository.

Collection expression arguments

You can pass arguments to the underlying collection's constructor or factory method by using a with(...) element as the first element in a collection expression. This feature enables you to specify capacity, comparers, or other constructor parameters directly within the collection expression syntax.

The following example shows how to pass a capacity argument to a List<T> constructor and a comparer to a HashSet<T>:

string[] values = ["one", "two", "three"];

// Pass capacity argument to List<T> constructor
List<string> names = [with(capacity: values.Length * 2), .. values];

// Pass comparer argument to HashSet<T> constructor
HashSet<string> set = [with(StringComparer.OrdinalIgnoreCase), "Hello", "HELLO", "hello"];
// set contains only one element because all strings are equal with OrdinalIgnoreCase

To learn more about collection expression arguments, see the language reference article on collection expressions or the feature specification. For information on using collection expression arguments in collection initializers, see Object and Collection Initializers.

Union types

C# 15 introduces union types, which represent a value that can be one of several case types. Declare a union with the union keyword:

public record class Cat(string Name);
public record class Dog(string Name);
public record class Bird(string Name);

public union Pet(Cat, Dog, Bird);

Unions provide implicit conversions from each case type, and the compiler ensures switch expressions are exhaustive across all case types:

Pet pet = new Dog("Rex");

string name = pet switch
{
    Dog d => d.Name,
    Cat c => c.Name,
    Bird b => b.Name,
};

Union types first appeared in .NET 11 Preview 2. In early .NET 11 previews, the UnionAttribute and IUnion interface aren't included in the runtime, so you must declare them in your project. Later .NET 11 preview versions include these runtime types. Also, some features from the proposal specification aren't yet implemented, including union member providers. Those features are coming in future previews.

For more information, see Union types in the language reference or the feature specification.