Visual Basic and Delphi Together, Creating a DLL

Khaled Shagrouni , Nov 2, 2001

Arabic

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This is an example of how to create a DLL in Dephi to be called from other development enviroment like Visual Basic.

The DLL uses TStringList, a powerful component in VCL Library, and wraps some of its properties and methods to manage list of strings.

The DLL code is listed below, also you can download the source and the compiled one with examples in Visual Basic and Delphi.

library TextDb;
{Example of a DLL}
{Always UnloadFile to free the memory}
uses
  SysUtils, Classes;

var
  List: TStringList;   {Global variable}

function LoadFile(lpString: PChar; Sorted: Cardinal): integer; stdcall;
begin
  try
    if List <> nil then List.Free; {Destroy any Previous instance}
    List := TstringList.create;    {Creating an instance to List}
    if Sorted > 0 then
    begin
      List.Sorted := true;
      List.Duplicates := dupAccept; {Allow duplicate values}
    end;
    if FileExists(lpString) then
      List.LoadFromFile(lpString);  {Load the file lines into our List}
    result := List.Count;           {Number of lines in the List}
  except                            {If any thing goes wrong..}
    List.Free;                      {Destroy the creature}
    List := nil;
    result := 0;
  end;
end;

function UnloadFile: integer; stdcall;
begin
  try
    if List <> nil then List.Free;
    List := nil;
    result := 1;
  except
    result := 0;
  end;
end;

function GetLine(lpString: PChar; nMaxCount, LineNum: Cardinal): integer; stdcall;
var
  s: string;
begin
  result := -1;
  if List <> nil then
    if Integer(LineNum) < List.Count then
    begin
      s := List[LineNum];
      StrLCopy(lpString, PChar(s), nMaxCount); {Convert Pascal string to PChar}
      result := Length(S);                     {Len(s)}
    end;
end;

function AddLine(lpString: PChar): integer; stdcall;
begin
  result := -1;
  if List <> nil then
  begin
    result := List.Add (lpString);         {return the position of the line}
  end;
end;

function SaveFile(lpString: PChar): integer; stdcall;
begin
  try
    if (List <> nil) and (Trim(lpString) <> '') then
      List.SaveToFile(lpString);           {Save lines into file}
    result := List.Count;                  {Number of lines in the List}
  except
    result := 0;
  end;
end;

function DeleteLine(LineNum: Cardinal): integer; stdcall;
begin
  result := -1;
  if (List <> nil) and (Integer(LineNum) < (List.Count)) then
  begin
    List.Delete(LineNum);
    result := List.Count;
  end;
end;

function UpdateLine(lpString: PChar; LineNum: Cardinal): integer; stdcall;
begin
  result := -1;
  if (List <> nil) and (Integer(LineNum) < (List.Count)) then
  begin
    List[LineNum] := lpString;
    result := LineNum;
  end;

end;

function LineCount: integer; stdcall;
begin
  result := -1;
  if List <> nil then
    result := List.Count;
end;

function GetText(lpString: PChar; nMaxCount: Cardinal): integer; stdcall;
var
  s: string;
begin
  result := -1;
  if List <> nil then
  begin
    s := List.Text;
    StrLCopy(lpString, PChar(s), nMaxCount); {Convert Pascal string to PChar}
    result := Length(S);                     {Len(s)}
  end;
end;

exports
  LoadFile, UnloadFile, GetLine, AddLine, DeleteLine, UpdateLine, SaveFile,
  LineCount, GetText;

begin
end.




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