如何从Delphi7中的基本数据模块继承

Question

I created a base datamodule as such:

unit dmi_Generic;
...
type
  TDMGenericClass = class of TdmiGeneric;

  TdmiGeneric = class(TDataModule)
private
  ReferencedForms: FormIDTypes;
public
  class procedure CloseDM(FormName: string; dm: TdmiGeneric);
  class function OpenDM(FormName: string; dm: TdmiGeneric; const dmClass:
                        TDMGenericClass): TdmiGeneric;
end;
...
class function TdmiGeneric.OpenDM(FormName: string; dm: TdmiGeneric; const
                dmClass: TDMGenericClass):
                TdmiGeneric;
var
    FormID: Integer;
begin
    FormID := GetFormID(FormName);
    if dm = nil then
        dm := dmClass.Create(nil);   //trouble is probably born here
    with dm do
        if not (FormID in ReferencedForms) then
            ReferencedForms := ReferencedForms + [FormID];
    Result := dm;
end;
...

I intended to inherit other datamodules from dmi_generic, so, in another unit...

Edit1

uses
    dmi_Generic;
...
type
  TdmUserSecurity = class(TdmiGeneric)
... 
var
  dmUserSecurity: TdmUserSecurity;

and in my main form...

uses
  UserSecurityDM;
  ...
procedure TfmMain.actUserManagerExecute(Sender: TObject);
begin
  dmUserSecurity := TdmUserSecurity.OpenDM(Self.Name,dmUserSecurity,TdmUserSecurity);
  //some code
  TdmUserSecurity.CloseDM(Self.Name,dmUserSecurity);
end;

Edit2: ...which gives compile error "Incompatible type. 'TdmUserSecurity' and 'TdmiGeneric'"

How do I fix this, properly? Do I typecast the return value at the main form, or is there something I should change at the dmi_generic unit? TIA

Answer 1

OpenDM() returns a TdmiGeneric pointer. But dmUserSecurity is not declared as TdmiGeneric , so you will get an error on this assignment:

dmUserSecurity := TdmUserSecurity.OpenDM(...);

You would need a type-cast to fix that, either:

dmUserSecurity := TdmUserSecurity(TdmUserSecurity.OpenDM(...));

Or:

dmUserSecurity := TdmUserSecurity.OpenDM(...) as TdmUserSecurity;

BTW, why does OpenDM() have a dm parameter at all? It is useless in the code you showed. You should either:

1. pass it by var and get rid of Result

    unit dmi_Generic; ... type TdmiGeneric = class; TDMGenericClass = class of TdmiGeneric; TdmiGeneric = class(TDataModule) private ... public ... class procedure CloseDM(FormName: string; var dm: TdmiGeneric); class procedure OpenDM(FormName: string; const dmClass: TDMGenericClass; var dm: TDMGeneric); end; ... class procedure TdmiGeneric.OpenDM(FormName: string; const dmClass: TDMGenericClass; var dm: TdmiGeneric); var ... begin ... if dm = nil then dm := dmClass.Create(nil); with dm do ... end; class procedure TdmiGeneric.CloseDM(FormName: string; var dm: TdmiGeneric); begin FreeAndNil(dm); end; 

    uses UserSecurityDM; ... procedure TfmMain.actUserManagerExecute(Sender: TObject); begin TdmUserSecurity.OpenDM(Self.Name, TdmUserSecurity, TdmiGeneric(dmUserSecurity)); //some code TdmUserSecurity.CloseDM(Self.Name, TdmiGeneric(dmUserSecurity)); end; 

2. get rid of dm as a parameter and declare it as a local variable instead:

    unit dmi_Generic; ... type TdmiGeneric = class; TDMGenericClass = class of TdmiGeneric; TdmiGeneric = class(TDataModule) private ... public ... class procedure CloseDM(FormName: string; var dm: TDMGeneric); class function OpenDM(FormName: string; const dmClass: TDMGenericClass): TDMGeneric; end; ... class function TdmiGeneric.OpenDM(FormName: string; const dmClass: TDMGenericClass): TdmiGeneric; var dm: TDMGeneric; ... begin ... dm := dmClass.Create(nil); with dm do ... Result := dm; end; class procedure TdmiGeneric.CloseDM(FormName: string; var dm: TDMGeneric); begin FreeAndNil(dm); end; 

    uses UserSecurityDM; ... procedure TfmMain.actUserManagerExecute(Sender: TObject); begin dmUserSecurity := TdmUserSecurity.OpenDM(Self.Name, TdmUserSecurity) as TdmUserSecurity; //some code TdmUserSecurity.CloseDM(Self.Name, TdmiGeneric(dmUserSecurity)); end; 

Either way, since you are calling OpenDM() on the TdmUserSecurity class and not on the TdmGeneric class, you might consider using the Self class type inside of OpenDM() , then you can remove the dmClass parameter:

unit dmi_Generic;
...
type
  TdmiGeneric = class;
  TDMGenericClass = class of TdmiGeneric;

  TdmiGeneric = class(TDataModule)
  private
    ...
  public
    ...
    class procedure OpenDM(FormName: string; var dm: TdmiGeneric);
  end;
...
class procedure TdmiGeneric.OpenDM(FormName: string; var dm: TdmiGeneric);
var
  ...
begin
  ...
  if dm = nil then
  begin
    if Self = TdmiGeneric then
      raise Exception.Create('Must call OpenDM() on a descendant class type').
    dm := TDMGenericClass(Self).Create(nil);
  end;
  with dm do
    ...
end;

uses
  UserSecurityDM;
...
procedure TfmMain.actUserManagerExecute(Sender: TObject);
begin
  TdmUserSecurity.OpenDM(Self.Name, TdmiGeneric(dmUserSecurity));
  ...
end;

Or:

unit dmi_Generic;
...
type
  TdmiGeneric = class;
  TDMGenericClass = class of TdmiGeneric;

  TdmiGeneric = class(TDataModule)
  private
    ...
  public
    ...
    class function OpenDM(FormName: string): TDMGeneric;
  end;
...
class function TdmiGeneric.OpenDM(FormName: string): TdmiGeneric;
var
  dm: TDMGeneric;
  ...
begin
  ...
  if Self = TdmiGeneric then
    raise Exception.Create('Must call OpenDM() on a descendant class type').
  dm := TDMGenericClass(Self).Create(nil);
  with dm do
      ...
  Result := dm;
end;

uses
  UserSecurityDM;
...
procedure TfmMain.actUserManagerExecute(Sender: TObject);
begin
  dmUserSecurity := TdmUserSecurity.OpenDM(Self.Name) as TdmUserSecurity;
  ...
end;