I'm trying this way, however the size remains 0
v := _RttiFld.GetValue(obj);
P := v.GetReferenceToRawData;
idx := 2; //to test
DynArraySetLength(p,v.TypeInfo,1,@idx);
Assert(_RttiFld.GetValue(obj).GetArrayLength > 0);
My field is a dynamic array of string
FMensage : array of string;