Delayed dynamic DLL loading

The problem: suppose you are writing a 3rd party library which uses an external DLL. This is the case when dynamic (explicit) DLL loading is preferable over static (implicit). Your library includes a load function which loads the DLL (by calling LoadLibrary and GetProcAddress functions) and now you have a dilemma – you can either warn the user that he should call the load function before using your library or you can call the load function yourself (in the initialization section of one of library units) so all a user should do is to place the DLL in the right location on disk. Both solutions have their pros and cons.

There is also an elegant solution that combines the best of both – you allow a user to load the DLL explicitly by calling the load function but if he did not done it the load function is called transparently when a DLL function is called first time. Here is an implementation example:

The DLL which exports 2 functions:

library DemoLib;

type
  TLoadResult = (LOAD_ERROR = -1, LOAD_OK = 0, LOAD_ALREADYLOADED = 1);

function GetValue(var Value: Integer): TLoadResult;
begin
  Value:= 42;
  Result:= LOAD_OK;
end;

function GetIsValid(AValue: Integer; var IsValid: Boolean): TLoadResult;
begin
  IsValid:= AValue = 42;
  Result:= LOAD_OK;
end;

exports
  GetValue,
  GetIsValid;

{$R *.res}

begin
end.

The load function:

unit LoadLib;

interface

type
  TLoadResult = (LOAD_ERROR = -1, LOAD_OK = 0, LOAD_ALREADYLOADED = 1);

type
  TGetValue = function(var Value: Integer): TLoadResult;
  TGetIsValid = function(AValue: Integer; var IsValid: Boolean): TLoadResult;

var
  GetValue: TGetValue;
  GetIsValid: TGetIsValid;

function LoadDemo(const Name: string = ''): TLoadResult;

implementation

uses Windows;

const
  LibName = 'DemoLib.dll';

function GetValueError(var Value: Integer): TLoadResult;
begin
  Result:= LOAD_ERROR;
end;

function GetIsValidError(AValue: Integer; var IsValid: Boolean): TLoadResult;
begin
  Result:= LOAD_ERROR;
end;

var
  LoadResult: TLoadResult = LOAD_OK;

function LoadDemo(const Name: string): TLoadResult;
var
  LibHandle: THandle;

begin
  if (LoadResult = LOAD_ALREADYLOADED) then begin
    Result:= LOAD_ALREADYLOADED;
    Exit;
  end;
  if Name = ''
    then LibHandle:= LoadLibrary(LibName)
    else LibHandle:= LoadLibrary(PChar(Name));
  if (LibHandle <> 0) then begin
    @GetValue:= GetProcAddress(LibHandle, 'GetValue');
    @GetIsValid:= GetProcAddress(LibHandle, 'GetIsValid');
    if (@GetValue <> nil) and (@GetIsValid <> nil) then begin
      LoadResult:= LOAD_ALREADYLOADED;
      Result:= LOAD_OK;
      Exit;
    end;
    FreeLibrary(LibHandle);
  end;
  @GetValue:= @GetValueError;
  @GetIsValid:= @GetIsValidError;
  LoadResult:= LOAD_ERROR;
  Result:= LOAD_ERROR;
end;

function GetValueStub(var Value: Integer): TLoadResult;
begin
  LoadDemo();
  Result:= GetValue(Value);
end;

function GetIsValidStub(AValue: Integer; var IsValid: Boolean): TLoadResult;
begin
  LoadDemo();
  Result:= GetIsValid(AValue, IsValid);
end;

initialization
  @GetValue:= @GetValueStub;
  @GetIsValid:= @GetIsValidStub;
end.

And the test application:

program TestLib;

{$APPTYPE CONSOLE}

uses
  SysUtils,
  LoadLib in 'LoadLib.pas';

procedure Test;
var
  Value: Integer;

begin
  if GetValue(Value) <> LOAD_OK then
    raise Exception.Create('Load Error');
  Writeln(Value);
end;

begin
  try
    Test;
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
  ReadLn;
end.

Initially GetValue and GetIsValid procedural variables reference GetValueStub and GetIsValidStub functions which call the DLL load function first; after the DLL was loaded these variables reference either the DLL functions if the loading was successful or the error functions GetValueError and GetIsValidError respectively if the loading failed.

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