Delphi Rtti for interfaces in a generic context

Question

for a framework I wrote a wrapper which takes any object, interface or record type to explore its properties or fields. The class declaration is as follows:

TWrapper<T> = class 
private
  FType : TRttiType;
  FInstance : Pointer;
  {...}
public
  constructor Create (var Data : T);
end;

In the constructor I try to get the type information for further processing steps.

constructor TWrapper<T>.Create (var Data : T);
begin
FType := RttiCtx.GetType (TypeInfo (T));
if FType.TypeKind = tkClass then
  FInstance := TObject (Data)
else if FType.TypeKind = tkRecord then
  FInstance := @Data
else if FType.TypeKind = tkInterface then
  begin
  FType := RttiCtx.GetType (TObject (Data).ClassInfo); //<---access violation
  FInstance := TObject (Data);
  end
else
  raise Exception.Create ('Unsupported type');
end;

I wonder if this access violation is a bug in delphi compiler (I'm using XE). After further investigation I wrote a simple test function, which shows, that asking for the class name produces this exception as well:

procedure TestForm.FormShow (Sender : TObject);
var
  TestIntf : IInterface;
begin
TestIntf    := TInterfacedObject.Create;
OutputDebugString(PChar (TObject (TestIntf).ClassName)); //Output: TInterfacedObject
Test <IInterface> (TestIntf);
end;

procedure TestForm.Test <T> (var Data : T);
begin
OutputDebugString(PChar (TObject (Data).ClassName)); //access violation
end;

Can someone explain me, what is wrong? I also tried the procedure without a var parameter which did not work either. When using a non generic procedure everything works fine, but to simplify the use of the wrapper the generic solution would be nice, because it works for objects and records the same way.

Kind regards,

Christian

Answer 1

Your code contains two wrong assumptions:

• That you can obtain meaningful RTTI from Interfaces. Oops, you can get RTTI from interface types.
• That a Interface is always implemented by a Delphi object (hence your attempt to extract the RTTI from the backing Delphi object).

Both assumptions are wrong. Interfaces are very simple VIRTUAL METHOD tables, very little magic to them. Since an interface is so narrowly defined, it can't possibly have RTTI. Unless of course you implement your own variant of RTTI, and you shouldn't. LE: The interface itself can't carry type information the way an TObject does, but the TypeOf() operator can get TypeInfo if provided with a IInterface

Your second assumption is also wrong, but less so. In the Delphi world most interfaces will be implemented by Delphi objects, unless of course you obtain the interface from a DLL written in an other programming language: Delphi's interfaces are COM-compatible, so it's implementations can be consumed from any other COM-compatible language and vice versa. But since we're talking Delphi XE here, you can use this syntax to cast an interface to it's implementing object in an intuitive and readable way:

TObject := IInterface as TObject;

that is, use the as operator. Delphi XE will at times automagically convert a hard cast of this type:

TObject := TObject(IInterface);

to the mentioned "as" syntax, but I don't like this magic because it looks very counter-intuitive and behaves differently in older versions of Delphi.

Casting the Interface back to it's implementing object is also wrong from an other perspective: It would show all the properties of the implementing object, not only those related to the interface, and that's very wrong, because you're using Interfaces to hide those implementation details in the first place!

Example: Interface implementation not backed by Delphi object

Just for fun, here's a quick demo of an interface that's not backed by an Delphi object. Since an Interface is nothing but an pointer to a virtual method table, I'll construct the virtual method table, create a pointer to it and cast the the pointer to the desired Interface type. All method pointers in my fake Virtual Method table are implemented using global functions and procedures. Just imagine trying to extract RTTI from my i2 interface!

program Project26;

{$APPTYPE CONSOLE}

uses
  SysUtils;

type

  // This is the interface I will implement without using TObject
  ITestInterface = interface
  ['{CFC4942D-D8A3-4C81-BB5C-6127B569433A}']
    procedure WriteYourName;
  end;

  // This is a sample, sane implementation of the interface using an
  // TInterfacedObject method
  TSaneImplementation = class(TInterfacedObject, ITestInterface)
  public
    procedure WriteYourName;
  end;

  // I'll use this record to construct the Virtual Method Table. I could use a simple
  // array, but selected to use the record to make it easier to see. In other words,
  // the record is only used for grouping.
  TAbnormalImplementation_VMT = record
    QueryInterface: Pointer;
    AddRef: Pointer;
    ReleaseRef: Pointer;
    WriteYourName: Pointer;
  end;

// This is the object-based implementation of WriteYourName
procedure TSaneImplementation.WriteYourName;
begin
  Writeln('I am the sane interface implementation');
end;

// This will implement QueryInterfce for my fake IInterface implementation. All the code does
// is say the requested interface is not supported!
function FakeQueryInterface(const Self:Pointer; const IID: TGUID; out Obj): HResult; stdcall;
begin
  Result := S_FALSE;      
end;

// This will handle reference counting for my interface. I am not using true reference counting
// since there is no memory to be freed, si I am simply returning -1
function DummyRefCounting(const Self:Pointer): Integer; stdcall;
begin
  Result := -1;
end;

// This is the implementation of WriteYourName for my fake interface.
procedure FakeWriteYourName(const Self:Pointer);
begin
  WriteLn('I am the very FAKE interface implementation');
end;

var i1, i2: ITestInterface;
    R: TAbnormalImplementation_VMT;
    PR: Pointer;

begin
  // Instantiate the sane implementation
  i1 := TSaneImplementation.Create;

  // Instantiate the very wrong implementation
  R.QueryInterface := @FakeQueryInterface;
  R.AddRef := @DummyRefCounting;
  R.ReleaseRef := @DummyRefCounting;
  R.WriteYourName := @FakeWriteYourName;
  PR := @R;
  i2 := ITestInterface(@PR);

  // As far as all the code using ITestInterface is concerned, there is no difference
  // between "i1" and "i2": they are just two interface implementations.
  i1.WriteYourName; // Calls the sane implementation
  i2.WriteYourName; // Calls my special implementation of the interface

  WriteLn('Press ENTER to EXIT');
  ReadLn;
end.

Answer 2

Two possible answers.

If this always happens, even when T is an object, then it's a compiler error and you ought to file a QC report about it. (With Interfaces, the cast-an-interface-to-an-object thing requires some black magic from the compiler, and it's possible that the generics subsystem doesn't implement it properly.)

If you're taking a T that's not an object, though, such as a record type, and getting this error, then everything's working as designed; you're just using typecasts improperly.

Either way, there's a way to get RTTI information out of any arbitrary type. You know how TRttiContext.GetType has two overloads? Use the other one. Instead of calling GetType (TObject (Data).ClassInfo), try calling GetType(TypeInfo(Data)).

Oh, and declare FInstance as a T instead of a pointer. It'll save you a lot of hassle.