Update csharp.md for Preview 1

release-notes/11.0/preview/preview1/csharp.md

@@ -1,8 +1,31 @@
 # C# in .NET 11 Preview 1 - Release Notes
 
-.NET 11 Preview 1 includes the following C# features & enhancements.
+.NET 11 Preview 1 includes two new features in C#:
 
-This Preview 1 release does not introduce new C# feature additions.
+- [Collection expression arguments](#collection-expression-arguments)
+- [Extended layout support](#extended-layout-support)
+
+## 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](https://learn.microsoft.com/dotnet/csharp/language-reference/operators/collection-expressions). The `with()` element must be the first element in the collection exprssion. You can specify values for any of the arguments to a specified constructor for the collection, as shown in the following example:
+
+```csharp
+// 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](https://learn.microsoft.com/dotnet/api/system.collections.generic.list-1.-ctor#system-collections-generic-list-1-ctor(system-int32)) 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](https://learn.microsoft.com/dotnet/csharp/whats-new/csharp-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](https://github.com/dotnet/roslyn/blob/main/docs/Language%20Feature%20Status.md#working-set-c), 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.
 
 C# updates: