RTTI: how to get the object pointer of a field? - delphi

Question

I have a TRttiProperty variable named aRttiProperty, that points to the property below:
Tsubscription = class(TMyObject)
private
fBilling: TMyObject;
public
property billing: TMyObject read fBilling; // << aRttiProperty point to this member
end;
Now, how can I extract the fBilling object pointer from aRttiProperty?
I try to do it like this, but it is not working:
function Tsubscription.getfBillingObj(const aRttiProperty: TRttiProperty): TMyObject
begin
Result := aRttiProperty.GetValue(Self).AsType<TMyObject>;
end;
It's returning the parent TSubscription object instead of the fbilling field object.

Answer 1

The code you showed in your question is perfectly fine (provided you fix your Tsubscription class declaration to include the getfBillingObj() method). The getfBillingObj() code you showed returns the correct object pointer, as demonstrated by the following code:
uses
System.Rtti;
type
TMyObject = class
public
Name: string;
constructor Create(const aName: string);
end;
Tsubscription = class(TMyObject)
private
fBilling: TMyObject;
public
constructor Create(const aName: string);
destructor Destroy; override;
function getfBillingObj(const aRttiProperty: TRttiProperty): TMyObject;
property billing: TMyObject read fBilling;
end;
constructor TMyObject.Create(const aName: string);
begin
inherited Create;
Name := aName;
end;
constructor Tsubscription.Create(const aName: string);
begin
inherited Create(aName);
fBilling := TMyObject.Create('bill');
end;
destructor Tsubscription.Destroy;
begin
fBilling.Free;
end;
function Tsubscription.getfBillingObj(const aRttiProperty: TRttiProperty): TMyObject;
begin
Result := aRttiProperty.GetValue(Self).AsType<TMyObject>;
end;
var
Ctx: TRttiContext;
prop: TRttiProperty;
sub: Tsubscription;
bill: TMyObject;
begin
sub := Tsubscription.Create('sub');
try
prop := ctx.GetType(Tsubscription).GetProperty('billing');
bill := sub.getfBillingObj(prop);
// bill.Name is 'bill' as expected...
finally
sub.Free;
end;
end;
That being said, it is not necessary to use RTTI in this situation since TSubscription has direct access to its own internal fields:
function TSubscription.getfBillingObj: TMyObject
begin
Result := fBilling;
end;
But even that is redundant since the billing property is public. Any caller can just use the billing property as-is:
var
sub: Tsubscription;
bill: TMyObject;
begin
sub := Tsubscription.Create('sub');
try
bill := sub.billing;
// bill.Name is 'bill' as expected...
finally
sub.Free;
end;
end;