C# in .NET 11 Preview 1 - Release Notes

.NET 11 Preview 1 includes two new features in C#:

• Collection expression arguments
• Extended layout support

C# updates:

• What's new in C# documentation
• Breaking changes

Collection expression arguments

This feature supports scenarios where a collection expression doesn't produce the collection type you need. You might want to pre-allocate storage because you know how you'll use the collection. For sorted collections, you may want to specify a different expression to compare items in the collection. For dictionaries, you may need a different expression to compare or find keys.

You specify arguments to the constructor of the collection in a with() element in the collection expression. The with() element must be the first element in the collection expression. You can specify values for any of the arguments to a specified constructor for the collection, as shown in the following example:

// Initialize to twice the capacity since we'll have to add
// more values later.
List<string> names = [with(capacity: values.Count * 2), .. values];

The compiler generates code to call the List<T> constructor with the capacity argument set to values.Count * 2.

You can learn more about this new feature in the What's new in C# 15 article. You'll find a longer description, and links to other documentation on the feature. This information includes information on how to specify arguments when the target type is an interface, such as IEnumerable<T>.

This feature will integrate with dictionary expressions, which is in progress now.

Extended layout support

The C# compiler emits the TypeAttributes.ExtendedLayout for types that have the System.Runtime.InteropServices.ExtendedLayoutAttribute applied. This is primarily intended for the .NET runtime team to use for types in interop scenarios.