Casting in generic class to interface delphi

Question

I'm getting a IEnumVariant from a .NET class library and I am trying to use a generic class to convert this to a IEnumerator

There is a compiler error, "Operator not applicable to this operand type" when attempting to cast an IInterface to the generic type T I've seen workarounds when attempting to type cast to a class, but these don't work for an interface.

Using Supports as suggested by Rob seems to have problems as well as TypeInfo returns nil for the parameterized type.

uses WinApi.ActiveX, Generics.Collections;

type
  TDotNetEnum<T: IInterface> = class(TInterfacedObject, IEnumerator<T>)
  strict private
    FDotNetEnum: IEnumVariant;
    FCurrent: T;
    function MoveNext: Boolean;
    procedure Reset;
    function GetCurrent: TObject;
    function IEnumerator<T>.GetCurrent = GenericGetCurrent;
    function GenericGetCurrent: T;
  public
    constructor Create(const ADotNetObject: OleVariant);

    //// I can get it to work using this constructor
    // constructor Create(const ADotNetObject: OleVariant; const AGUID: TGUID);
  end;

implementation

uses System.Rtti, SysUtils, mscorlib_TLB, ComObj;

constructor TDotNetEnum<T>.Create(const ADotNetObject: OleVariant);
var
  netEnum: IEnumerable;
begin
  netEnum := IUnknown(ADotNetObject) as mscorlib_TLB.IEnumerable;
  FDotNetEnum := netEnum.GetEnumerator();
end;

function TDotNetEnum<T>.GenericGetCurrent: T;
begin
  result := FCurrent;
end;

function TDotNetEnum<T>.GetCurrent: TObject;
begin
  result := nil;
end;

function TDotNetEnum<T>.MoveNext: Boolean;
var
  rgvar: OleVariant;
  fetched: Cardinal;
  ti: TypeInfo;
  guid: TGUID;
begin
  OleCheck(FDotNetEnum.Next(1, rgvar, fetched));
  result := fetched = 1;
  if not result then
    FCurrent := nil
  else
  begin
    FCurrent := IUnknown(rgvar) as T; // <-- Compiler error here
    //// Doesn't work using Supports either
    // ti := TypeInfo(T);  // <-- returns nil
    // guid := GetTypeData(@ti)^.Guid;
    // Supports(IUnknown(rgvar), guid, FCurrent);
  end;
end;

procedure TDotNetEnum<T>.Reset;
begin
  OleCheck(FDotNetEnum.Reset);
end;

Am I missing something in order to get that case to the generic interface type to work ?

I do have the alternative constructor which I CAN get the guid from so that

TDotNetEnum<IContact>.Create(vContactList, IContact);

works but the ideal

TDotNetEnum<IContact>.Create(vContactList); 

doesn't

Answer 1

Using as to cast interfaces is only valid for interfaces that have GUIDs. The compiler cannot assume that T has a GUID when it's compiling your generic class, so it cannot accept an expression of the form val as T.

This has been covered before, but in reference to the Supports function, which has the same limitation as the as operator.

The solution is to use RTTI to fetch the interface's GUID, and then use that to type-cast the interface value. You could use Supports:

guid := GetTypeData(TypeInfo(T))^.Guid;
success := Supports(IUnknown(rgvar), guid, FCurrent);
Assert(success);

You could also call QueryInterface directly:

guid := GetTypeData(TypeInfo(T))^.Guid;
OleCheck(IUnknown(rgvar).QueryInterface(guid, FCurrent));