Attribute validation and Overflow

This table lists how overflow is "encoded" in the POCO model, and which validator will catch the error.

Kind of error	How encoded	Detected by(1)
Incorrect primitive value	Incorrect value is stored in primitive's ObjectValue	ValidateObjectValue()
Expected primitive, got object	(Dynamic)object is stored in primitive's ObjectValue	ValidateObjectValue()
Expected object, got primitive	Store in overflow, with correct type "attached" (2)	SetValue(3)+ValidateObject
Expected single, got array	Store in overflow, with correct type "attached" (2)	SetValue(3)+ValidateObject
Expected array, got single	Store in overflow, with correct type "attached" (2)	SetValue(3)+ValidateObject
Choice where no choice allowed	Added as unknown element to overflow	See Unknown element below
Invalid choice type	Property (type of DataType) is set to the value	AllowedTypes attribute
Unknown choice type	Property is set to a DynamicDataType value	AllowedTypes attribute
Unknown resource type	Property is set to a DynamicResource value	AllowedTypes attribute (4)
Unknown element	Store in overflow, with "no type" attached	SetValue(3)+ValidateObject

(1) The property's getter (except for JsonValue) also detects it, and throws. We still might need an unsafe marker to do this efficiently. The setter needs to detect invalid values to remove any entries from the overflow dictionary when a new value is set.

(2) This probably means the overflow needs to contain tuples or types which allow for a value + expected type.

(3) We need detection in ValidateObject for .NET attribute validation, but in either SetValue or TryGetValue for the parser, since the parsers need to be able to validate each property as they parse them, calling ValidateObject is too late (no more position info available). This probably means we need to change the signature of SetValue or TryGetValue to also be able to get the error. Or we add an explicit ValidateProperty(name) to the generated code. Or we let it be detected by ValidateObject for attribute validation, and by the json/xml parser during parsing (two separate detections paths).

(4) Normally, nested resources are marked with [AllowedTypes(typeof(Hl7.Fhir.Model.Resource))], we should make sure that in that case we complain about finding a DynamicResource.