`ref struct` types

• added as part of C# 7.2.
• defined with ref struct StructName
• always on the stack
• never part of heap
• cannot be element of an array
• cannot be field type of reference type
• can't implement interfaces
• can't be boxed
• can't be type argument
• can't be captured by closure of lambda or local function
• can't be used in async methods
• can't be used in iterators

Further read:

https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/ref-struct