有人在使用QueryInterface在Delphi中注意到这种行为吗？

Question

这是我的类型......

unit unitTestInterfaces;

interface
type
  IFoo = interface
    ['{169AF568-4568-429A-A8F6-C69F4BBCC6F0}']
    function TestFoo1:string;
    function TestFoo:string;
  end;

  IBah = interface
    ['{C03E4E20-2D13-45E5-BBC6-9FDE12116F95}']
    function TestBah:string;
    function TestBah1:string;
  end;

  TFooBah = class(TInterfacedObject, IFoo, IBah)
    //IFoo
    function TestFoo1:string;
    function TestFoo:string;

    //IBah
    function TestBah1:string;
    function TestBah:string;
  end;

implementation

{ TFooBah }

function TFooBah.TestBah: string;
begin
  Result := 'TestBah';
end;

function TFooBah.TestBah1: string;
begin
  Result := 'TestBah1';
end;

function TFooBah.TestFoo: string;
begin
  Result := 'TestFoo';
end;

function TFooBah.TestFoo1: string;
begin
  Result := 'TestFoo1';
end;

end.

这是运行示例的代码...

var
  fb:TFooBah;
  f:IFoo;
  b:IBah;
begin
  try
    fb := TFooBah.Create;

    /// Notice we've used IBah here instead of IFoo, our writeln() still outputs the
    /// result from the TestBah() function, presumably because it's the "first" method
    /// in the IBah interface, just as TestFoo1() is the "first" method in the IFoo
    /// interface.
    (fb as IUnknown).QueryInterface(IBah,f); //So why bother with this way at all??
    //f := fb as IBah; //causes compile error
    //f := fb; //works as expected
    if Assigned(f)then
    begin
      writeln(f.TestFoo1); //wouldn't expect this to work since "f" holds reference to IBah, which doesn't have TestFoo1()
    end;

    (fb as IUnknown).QueryInterface(IBah,b);
    if Assigned(f) then
    begin
      writeln(b.TestBah1);
    end;

  except on E:Exception do
    writeln(E.Message);
  end;
end.

似乎在第一次调用QueryInterface时,即使我们将错误类型的接口分配给"f"变量,它仍将尝试执行它所指向的任何"第一"方法,而不是使用名称"TestFoo1".使用f:= fb按预期工作,所以有理由我们使用QueryInterface而不是语法f:= fb吗？

Answer 1

我想你在违反规则:

QueryInterface会将接口放入您请求的f中.你有责任f是适当的类型.由于第二个参数是无类型的,编译器无法警告您的错误.