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TFunction = procedure of Object;
VarFunction: TFunction;

procedure MachWas;
begin
  showmessage('Hallo');
end;

procedure VarFunctionRuecksetzen;
begin
  VarFunction:= nil;
end;

procedure VarFunctionSetzen;
begin
  VarFunction:= MachWas;
end;

procedure VarFunctionAusfuehren;
begin
  if VarFunction <> nil then VarFunction; //hier ist das problem, "incompatible Types"
end;					

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procedure VarFunctionRuecksetzen;
begin
  VarFunction:= doNothing;
end;

procedure doNothing;
begin
  //
end;

procedure VarFunctionAusfuehren;
begin
  doNothing;
end;					

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@VarFunction <> nil					

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function Assigned(const P): Boolean;					

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...
if Assigned(FOnClick) then
  FOnClick(Self);
...