Collection expressions ^{C# 12.0} gave us a concise [...] syntax for creating collections, but it had no way to choose how the underlying collection was constructed. If you needed a capacity or a custom comparer you had to fall back to a constructor call.

C# 15 lets you place a with(...) element first in a collection expression to pass arguments to the collection's constructor or factory method.

Code

Now

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

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

// Comparer passed to the HashSet<T> constructor
HashSet<string> set = [with(StringComparer.OrdinalIgnoreCase), "Hello", "HELLO", "hello"];
// set contains one element as all three are equal under OrdinalIgnoreCase

Before

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

List<string> names = new(values.Length * 2);
names.AddRange(values);

HashSet<string> set = new(StringComparer.OrdinalIgnoreCase) { "Hello", "HELLO", "hello" };

Notes

• The with(...) element must be the first element in the collection expression
• Arguments can be positional or named, such as with(capacity: 100)
• Arrays and spans do not accept with(...), not even an empty with()
• Collection types with no parameterless constructor can now be targeted, as long as the expression includes with(...)
• If you have a method called with, call it as @with(...) inside a collection expression

More information

• What's new in C# 15.0
• C# Reference
• Feature specification