Can Delphi only use a .dll if required?

This facility, known as delay loading, was added in Delphi 2010.

Using your code as an example you could write your import like this:

function Inp(PortAddress: Integer): Integer; stdcall; 
    external 'inpout32.dll' name 'Inp32' delayed;

The binding to this external function will be performed only when the function is first called. If the binding fails then an exception is raised at runtime.

You can use SetDliNotifyHook and SetDliFailureHook to customise the delay loading behaviour should you need even more fine-grained control.

Some blog articles to supplement the product documentation:

• Allen Bauer: Procrastinators Unite… Eventually!
• Allen Bauer: Exceptional Procrastination
• Dr. Bob: Delphi 2010 Delayed Dynamic Link Libraries

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On older versions of Delphi you can use LoadLibrary and GetProcAddress. Or, if you want something a little slicker I can heartily recommend Hallvard Vassbotn's delay load class which he describes in this blog article. This code wraps up all the boiler plate of calling LoadLibrary and GetProcAddress and is only slightly more cumbersome to use than the new Delphi 2010 built-in feature.

I successfully used Hallvard's library for many years. One minor word of caution is that it is not threadsafe so if multiple threads attempt to bind to a function at the same time then the code can fail. This is easy enough to fix by adding internal locks to Hallvard's code.

In Delphi versions prior to 2010, you have to use classic dynamic loading. Consider this typical (and simple) example calling the Beep function from Kernel32.dll (which you should not hardcode the path to in real code, of course!):

type
  TBeepFunc = function(dwFreq: DWORD; dwDuration: DWORD): BOOL; stdcall;

procedure TForm4.FormClick(Sender: TObject);
var
  lib: HMODULE;
  prc: TBeepFunc;
begin

  lib := LoadLibrary('C:\WINDOWS\System32\Kernel32.dll');
  if lib = 0 then RaiseLastOSError;
  try
    @prc := GetProcAddress(lib, 'Beep');
    if Assigned(prc) then
      prc(400, 2000)
    else
      ShowMessage('WTF? No Beep in Kernel32.dll?!');
  finally
    FreeLibrary(lib);
  end;
end;