I've recently experimented extensively with interfaces and D2010 RTTI. I don't know at runtime the actual type of the interface; although I will have access to it's qualified name using a string.

Consider the following:

program rtti_sb_1;
{$APPTYPE CONSOLE}
uses
  SysUtils, Rtti, TypInfo, mynamespace in 'mynamespace.pas';
var
  ctx:                  TRttiContext;
  InterfaceType:        TRttiType;
  Method:               TRttiMethod;
  ActualParentInstance: IParent;
  ChildInterfaceValue:  TValue;
  ParentInterfaceValue: TValue;
begin
  ctx := TRttiContext.Create;
  // Instantiation
  ActualParentInstance := TChild.Create as IParent;
  {$define WORKAROUND}
  {$ifdef WORKAROUND}
  InterfaceType := ctx.GetType(TypeInfo(IParent));
  InterfaceType := ctx.GetType(TypeInfo(IChild));
  {$endif}
  // Fetch interface type
  InterfaceType := ctx.FindType('mynamespace.IParent');
  // This cast is OK and ChildMethod is executed
  (ActualParentInstance as IChild).ChildMethod(100);
  // Create a TValue holding the interface
  TValue.Make(@ActualParentInstance, InterfaceType.Handle, ParentInterfaceValue);
  InterfaceType := ctx.FindType('mynamespace.IChild');
  // This cast doesn't work
  if ParentInterfaceValue.TryCast(InterfaceType.Handle, ChildInterfaceValue) then begin
    Method := InterfaceType.GetMethod('ChildMethod');
    if (Method <> nil) then begin
      Method.Invoke(ChildInterfaceValue, [100]);
    end;
  end;
  ReadLn;
end.

The contents of mynamespace.pas is as follows:

{$M+}
IParent = interface
  ['{2375F59E-D432-4D7D-8D62-768F4225FFD1}']
  procedure ParentMethod(const Id: integer);
end;
{$M-}
IChild = interface(IParent)
  ['{6F89487E-5BB7-42FC-A760-38DA2329E0C5}']
  procedure ChildMethod(const Id: integer);
end;
TParent = class(TInterfacedObject, IParent)
public
  procedure ParentMethod(const Id: integer);
end;
TChild = class(TParent, IChild)
public
  procedure ChildMethod(const Id: integer);
end;

For completeness, the implementation goes as

procedure TParent.ParentMethod(const Id: integer);
begin
  WriteLn('ParentMethod executed. Id is ' + IntToStr(Id));
end;
procedure TChild.ChildMethod(const Id: integer);
begin
  WriteLn('ChildMethod executed. Id is ' + IntToStr(Id));
end;

The reason for {$define WORKAROUND} may be found in this post.

Question: is there any way for me to make the desired type cast using RTTI? In other words: is there a way for me to invoke IChild.ChildMethod from knowing 1) the qualified name of IChild as a string, and 2) a reference to the TChild instance as a IParent interface? (After all, the hard-coded cast works fine. Is this even possible?) Thanks!