Currently it is not possible to apply attributes to the backing fields of auto-implemented properties. In those cases where the developer must use a field-targeting attribute they are forced to declare the field manually and use the more verbose property syntax. Given that C# has always supported field-targeted attributes on the generated backing field for events it makes sense to extend the same functionality to their property kin.
There are two potential drawbacks to implementing this change:
1. Attempting to apply an attribute to the field of an auto-implemented property produces a compiler warning that the attributes in that block will be ignored. If the compiler were changed to support those attributes they would be applied to the backing field on a subsequent recompilation which could alter the behavior of the program at runtime.
1. The compiler does not currently validate the AttributeUsage targets of the attributes when attempting to apply them to the field of the auto-implemented property. If the compiler were changed to support field-targeted attributes and the attribute in question cannot be applied to a field the compiler would emit an error instead of a warning, breaking the build.