How to make call to DLL function from Delphi? - delphi

Question

// Get a list of accounts in a domain separated by \x00 and ended by \x00\x00
Function GetUserList(AName: PAnsiChar; Var List; Size: Longint): Longint; StdCall;
I need to call the above from XE6.
Would someone be kind enough to post an example of how I can
get this buffer, and put it to a stream or a string.
The variable "List" is supposed to fill up some buffer, which I can read
off the list of users.
After trying for a couple of options, I have tried all options such as:
thanks!
var
Buffer: array of Byte;
iCount : Integer;
sName : AnsiString;
begin
...
SetLength(Buffer, 4096);
iCount := GetUserListTest(PAnsiChar(sName)#Buffer[0], Length(Buffer)); // cannot
// iCount := GetUserList(PAnsiChar(sName), Buffer, Length(Buffer));

Answer 1

That is not a Win32 API function, so it must be a third-party function. Ask the vendor for an example.
A var parameter expects you to pass a variable to it. The var receives the address of the variable. #Buffer[0] does not satisfy that requirement, as # returns a Pointer, and then the var ends up with the address of the pointer itself, not the address of the variable being pointed at. The function is expecting a pointer to a buffer. By using a var to receive that pointer, you need to drop the # and pass the first array element, so that the address of that element (effectively the address of the buffer) will be passed to the function, eg:
iCount := GetUserList(PAnsiChar(sName), Buffer[0], iCount);
Alternatively, you can use this syntax instead, which will pass the same address of the first element:
iCount := GetUserList(PAnsiChar(sName), PByte(Buffer)^, iCount);
Now, with that said, chances are that the function may allow you to query it for the necessary array size so you can allocate only what is actually needed (but check the documentation to be sure, I'm making an assumption here since you have not said otherwise)), eg:
procedure GetDomainUsers(const Domain: AnsiString; Users: TStrings);
var
Buffer: array of AnsiChar;
iCount : Integer;
User: PAnsiChar;
begin
// this call ASSUMES the function returns the needed
// bytecount when given a NULL/empty array - check
// the documentation!!!
iCount := GetUserList(PAnsiChar(Domain), PAnsiChar(nil)^, 0);
if iCount > 0 then
begin
SetLength(Buffer, iCount);
iCount := GetUserList(PAnsiChar(Domain), Buffer[0]{or: PAnsiChar(Buffer)^}, iCount);
end;
if iCount > 0 then
begin
Users.BeginUpdate;
try
User := PAnsiChar(Buffer);
while User^ <> #0 do
begin
Users.Add(User);
Inc(User, StrLen(User)+1);
end;
finally
Users.EndUpdate;
end;
end;
end;
If that does not work, then you will have to pre-allocate a large array:
procedure GetDomainUsers(const Domain: AnsiString; Users: TStrings);
var
Buffer: array of AnsiChar;
User: PAnsiChar;
begin
SetLength(Buffer, 1024);
if GetUserList(PAnsiChar(Domain), Buffer[0]{or: PAnsiChar(Buffer)^}, Length(Buffer)) > 0 then
begin
Users.BeginUpdate;
try
User := PAnsiChar(Buffer);
while User^ <> #0 do
begin
Users.Add(User);
Inc(User, StrLen(User)+1);
end;
finally
Users.EndUpdate;
end;
end;
end;