Delphi interface type incompatibility

user_243837 0
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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.

delphi
1 Answers

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.

user_658630
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