Software1 hOctober 16, 2026, 08:41

C# 15 adds with() for collection constructor arguments

C# 15, shipping with .NET 11, introduces a with(...) element in collection expressions whose arguments go to the constructor or factory, such as a HashSet comparer. The feature is available in .NET 11 RC1, released September 8, 2026 with a go-live license.

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C# 15 adds with() for collection constructor arguments

with(...) must be the first element inside the brackets
Arguments are passed to the constructor or [CollectionBuilder] factory
Arrays and spans reject with() with error CS9355
Dictionary accepts a comparer but not KeyValuePair elements

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C# 15 adds with() for collection constructor arguments

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