How do I cast a TObject as a TObjectList<T>? - delphi

Question

I have a procedure that needs to insert an array of TObjects into to a list. The list can be of any of the supported types, e.g. TObjectList, TObjectList<T>, TROArray, etc.
The procedure looks like this:
type
TObjectArray = Array of TObject;
...
procedure TMyClass.DoAssignObjectList(const ObjectArray: TObjectArray;
const DstList: TObject);
var
i: Integer;
begin
if DstList is TObjectList then
begin
for i := 0 to pred(TObjectList(DstList).Count) do
TObjectList(DstList).Add(ObjectArray[i]);
end else
if DstList is TObjectList<T> then // Obviously this doesn't work
begin
for i := 0 to pred(TObjectList<T>(DstList).Count) do
TObjectList<T>(DstList).Add(ObjectArray[i]);
end
else
begin
raise Exception.CreateFmt(StrNoDoAssignORMObject, [DstList.ClassName]);
end;
end;
How can I check that an object is a TObjectList<T> and then add the elements of an array to it?

Answer 1

You have to use a bit RTTI to get some more information about the generic type.
The following code uses Spring4D which has some methods for that:
uses
...
Spring.Reflection;
procedure DoAssignObjectList(const ObjectArray: TObjectArray;
const DstList: TObject);
function IsGenericTObjectList(const obj: TObject): Boolean;
var
t: TRttiType;
begin
t := TType.GetType(obj.ClassInfo);
Result := t.IsGenericType and (t.GetGenericTypeDefinition = 'TObjectList<>');
end;
begin
...
if IsGenericTObjectList(DstList) then
begin
for i := 0 to pred(TObjectList<TObject>(DstList).Count) do
TObjectList<TObject>(DstList).Add(ObjectArray[i]);
...
end;
Additionally to that you can also get information about the generic parameter type of the list to check if the objects you are putting into it are matching the requirements (only works on a generic type of course):
function GetGenericTObjectListParameter(const obj: TObject): TClass;
var
t: TRttiType;
begin
t := TType.GetType(obj.ClassInfo);
Result := t.GetGenericArguments[0].AsInstance.MetaclassType;
end;

Answer 2

As I was writing this question I figured out a way to do this using RTTI. It should work with any list that has a procedure Add(AObject: TObject).
procedure TransferArrayItems(const Instance: TObject;
const ObjectArray: TObjectArray);
const
AddMethodName = 'Add';
var
Found: Boolean;
LMethod: TRttiMethod;
LIndex: Integer;
LParams: TArray<TRttiParameter>;
i: Integer;
RTTIContext: TRttiContext;
RttiType: TRttiType;
begin
Found := False;
LMethod := nil;
if length(ObjectArray) > 0 then
begin
RTTIContext := TRttiContext.Create;
RttiType := RTTIContext.GetType(Instance.ClassInfo);
for LMethod in RttiType.GetMethods do
begin
if SameText(LMethod.Name, AddMethodName) then
begin
LParams := LMethod.GetParameters;
if length(LParams) = 1 then
begin
Found := TRUE;
for LIndex := 0 to length(LParams) - 1 do
begin
if LParams[LIndex].ParamType.Handle <> TValue(ObjectArray[0]).TypeInfo
then
begin
Found := False;
Break;
end;
end;
end;
if Found then
Break;
end;
end;
if Found then
begin
for i := Low(ObjectArray) to High(ObjectArray) do
begin
LMethod.Invoke(Instance, [ObjectArray[i]]);
end;
end
else
begin
raise Exception.CreateFmt(StrMethodSNotFound, [AddMethodName]);
end;
end;
end;