Delphi interface type incompatibility-CodePudding

What I am trying to do is to add common methods to these two classes which have the same indirect ancestor.

IMyMethods = interface
  procedure SomeMethod;
end;

TMyADODataset =class(TADODataset, IMyMethods) // ADO
public
  procedure SomeMethod;
end;

TMyUniDataset =class(TUniTable, IMyMethods) // UniDAC
public
  procedure SomeMethod;
end;

SomeMethod would be implemented differently for ADO and for UniDAC. So I thought an interface is perfect.

Then we have

TMyTable =class
private
  FDataset: TDataset;
end;

Here I have choosen TDataset as that is the common ancestor of TADODataset and TUniTable.

FDataset could be instantiated as follows:

if FProvider = prADO then
  FDataset := TMyADODataset.Create
else
  FDataset := TMyUniDataset.Create;

Now the problem is how to call SomeMethod of FDataset, the following does not compile and gives a type incompatibility error:

IMyMethods(FDataset).SomeMethod;

This is because TDataset does not implement IMyMethods, which is correct. But is there any way I can trick the compiler into accepting this? Or is there a better solution? I thought of class helpers but the implentation of SomeMethod will be different for ADO and UniDAC.

CodePudding user response：

Use the SysUtils.Supports() function to obtain the IMyMethods interface from the FDataset object, eg:

uses
  ..., SysUtils;

var
  Intf: IMyMethods;
...
if Supports(FDataset, IMyMethods, Intf) then
  Intf.SomeMethod;

Just note that, in order for this to work, IMyMethods needs to have a Guid assigned to it, eg:

type
  IMyMethods = interface
    ['{xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}']
    procedure SomeMethod;
  end;

You can generate a new Guid directly in the Code Editor by pressing Ctrl Shift G.