Errors and warnings associated with ref struct types

• CS8343: ref structs cannot implement interfaces
• CS8344: foreach statement cannot operate on enumerators in async or iterator methods because type is a ref struct or a type parameter that allows ref struct.
• CS8345: Field or auto-implemented property cannot be of type unless it is an instance member of a ref struct.
• CS9048: The scoped modifier can be used for refs and ref struct values only.
• CS9050: A ref field cannot refer to a ref struct.
• CS9059: A ref field can only be declared in a ref struct.
• CS9241: 'ref struct' is already specified.
• CS9242: The 'allows' constraint clause must be the last constraint specified.
• CS9243: Cannot allow ref structs for a type parameter known from other constraints to be a class.
• CS9244: The type may not be a ref struct or a type parameter allowing ref structs in order to use it as parameter in the generic type or method.
• CS9245: Type cannot implement interface member for ref struct type.
• CS9246: A non-virtual instance interface member cannot be accessed on a type parameter that allows ref struct.
• CS9247: foreach statement cannot operate on enumerators of type because it is a type parameter that allows ref struct and it is not known at compile time to implement IDisposable.
• CS9267: Element type of an iterator may not be a ref struct or a type parameter allowing ref structs

ref safety violations

• CS8345: Field or auto-implemented property cannot be of type unless it is an instance member of a ref struct.
• CS9048: The scoped modifier can be used for refs and ref struct values only.
• CS9050: A ref field cannot refer to a ref struct.
• CS9059: A ref field can only be declared in a ref struct.

A ref struct type can include ref fields. Other types aren't allowed ref fields. The compiler enforces restrictions on the declarations and use of ref struct types to enforce ref safety rules on instances of any ref struct type:

• Only ref struct types can contain automatically implemented ref properties.
• Only ref struct types or ref variables can have the scoped modifier.
• A ref field can be declared only in a ref struct type.
• A ref field can't refer to a ref struct type/

Violating any of these rules produces one of the listed errors. If you intended to use that language feature, convert the type to a ref struct. Otherwise, remove the disallowed construct.

ref struct interface implementations

• CS8343: ref structs cannot implement interfaces
• CS8344: foreach statement cannot operate on enumerators in async or iterator methods because type is a ref struct or a type parameter that allows ref struct.
• CS9241: 'ref struct' is already specified.
• CS9242: The 'allows' constraint clause must be the last constraint specified.
• CS9243: Cannot allow ref structs for a type parameter known from other constraints to be a class.
• CS9244: The type may not be a ref struct or a type parameter allowing ref structs in order to use it as parameter in the generic type or method.
• CS9245: Type cannot implement interface member for ref struct type.
• CS9246: A non-virtual instance interface member cannot be accessed on a type parameter that allows ref struct.
• CS9247: foreach statement cannot operate on enumerators of type because it is a type parameter that allows ref struct and it is not known at compile time to implement IDisposable.
• CS9267: Element type of an iterator may not be a ref struct or a type parameter allowing ref structs

Prior to C# 13, ref struct types can't implement interfaces; the compiler generates CS8343. Beginning with C# 13, ref struct types can implement interfaces, subject to the following rules:

• A ref struct can't be converted to an instance of an interface it implements. This restriction includes the implicit conversion when you use a ref struct type as an argument when the parameter is an interface type. The conversion results in a boxing conversion, which violates ref safety.
• A ref struct that implements an interface must implement all interface members. The ref struct must implement members where the interface includes a default implementation.

Beginning with C# 13, a ref struct can be used as a type argument for a generic type parameter, if and only if the generic type parameter has the allows ref struct anti-constraint. When you use the allows ref struct anti-constraint you must follow these rules:

• A ref struct is used as a type argument, the type parameter must have the allows ref struct anti-constraint.- The allows ref struct anti-constraint must be last in the where clause for that parameter
• Uses of instances the type parameter must obey ref safety rules.
• A ref struct or a type argument that can be a ref struct type can't be used as the element type for an iterator method.