Avoiding code duplication in Delphi - delphi

Question

I have two components A and B. Component B derives from component A and shares most properties and procedures with it. Now I have a lengthy procedure like this:
procedure DoSomething;
begin
Form1.Caption := Component_A.Caption;
// hundreds of additional lines of code calling component A
end;
Depending on whether component B is active or not, I would like to reuse the above procedure and replace the Component_A part with the name of component B. It should look like this then:
procedure DoSomething;
var
C: TheComponentThatIsActive;
begin
if Component_A.Active then
C := Component_A;
if Component_B.Active then
C := Component_B;
Form1.Caption := C.Caption;
end;
How can I do that in Delphi2007?
Thanks!

Answer 1

TheComponentThatIsActive should be the same type that ComponentA is (TComponentA).
Now, if you run into a stumbling block where some properties/methods only belong to ComponentB, then check and typecast it.
procedure DoSomething;
var
C: TComponentA;
begin
if Component_A.Active then
C := Component_A
else if Component_B.Active then
C := Component_B
else
raise EShouldNotReachHere.Create();
Form1.Caption := C.Caption;
if C=Component_B then
Component_B.B_Only_Method;
end;

Answer 2

You can pass ComponentA or ComponentB to DoSomething as a parameter.
ComponentA = class
public
procedure Fuu();
procedure Aqq();
end;
ComponentB = class(ComponentA)
public
procedure Blee();
end;
implementation
procedure DoSomething(context:ComponentA);
begin
context.Fuu();
context.Aqq();
end;
procedure TForm1.Button1Click(Sender: TObject);
var cA:ComponentA;
cB:ComponentB;
begin
cA:= ComponentA.Create();
cB:= ComponentB.Create();
DoSomething(cA);
DoSomething(cB);
cA.Free;
cB.Free;
end;