Are interface methods always virtual?

Question

When compiling the following code I get an error:

TOmniParallelSimplePooledLoop = class(TOmniParallelSimpleLoop)
  procedure Execute(loopBody: TOmniIteratorSimpleSimpleDelegate); overload; override;

[dcc64 Error] OtlParallel.pas(846): E2170 Cannot override a non-virtual method

If I make the ancestor method virtual then the error goes away.

However the ancestor method is declared in:

IOmniParallelSimpleLoop
  ...
  procedure Execute(loopBody: TOmniIteratorSimpleSimpleDelegate); overload;

Will the redeclaration of the base method in TOmniParallelSimpleLoop from non-virtual to virtual change the base type, or was the method already virtual to begin with (due to it being an implementation of an interface method)?

In other words: will the compiler output different code when an interfaced method is changed from non-virtual to virtual?

Basic MSVC to recreate the error

program Project70;
{$APPTYPE CONSOLE}
uses
  System.SysUtils;

type
  I1 = interface
    procedure DoSomething;
  end;

  T1 = class(TInterfacedObject, I1)
    procedure DoSomething;
  end;

  T2 = class(T1)
    procedure DoSomething; override;
  end;

procedure T1.DoSomething;
begin
  WriteLn('parent');
end;

procedure T2.DoSomething;
begin
  Writeln('Child');
end;

begin
end.

Are interface methods always virtual?

2 Answers