Inno Setup URLDownloadToCacheFile - inno-setup

Question

Any time i try to compile this code, a get an error msg -> Type mismatch. Why?
The first variant is called with the same "nil" and the compiler has no problem with it, but with the last?
[Setup]
AppName=MyApp
AppVerName=MyApp
DefaultDirName={pf}\MyApp
DefaultGroupName=MyApp
OutputDir=.
[Code]
function URLDownloadToCacheFile(const lpUnkcaller: variant; const szURL: PAnsiChar; var szFileName: PAnsiChar; const cchFileName: DWORD; const dwReserved: DWORD; IBindStatusCallback: variant): HRESULT; external 'URLDownloadToFileW#Urlmon.dll stdcall';
function InitializeSetup(): Boolean;
var
szFileName, szURL : PAnsiChar;
ErrorCode : HRESULT;
MAX_PATH : DWORD;
begin
szURL := 'http://fs2.directupload.net/images/user/150607/x3sugvzy.jpg';
MAX_PATH := 512;
SetLength(szFileName, MAX_PATH + 1);
ErrorCode := URLDownloadToCacheFile(null, szURL, szFileName, MAX_PATH, 0, null);
end;
changed code that way:
[Setup]
AppName=MyApp
AppVerName=MyApp
DefaultDirName={pf}\MyApp
DefaultGroupName=MyApp
OutputDir=.
[Code]
function URLDownloadToCacheFile(const lpUnkcaller: variant; const szURL: AnsiString; var szFileName: String; const cchFileName: DWORD; const dwReserved: DWORD; IBindStatusCallback: variant): HRESULT; external 'URLDownloadToFileW#Urlmon.dll stdcall';
function InitializeSetup(): Boolean;
var
szFileName : String;
szURL : AnsiString;
ErrorCode : HRESULT;
MAX_PATH : DWORD;
begin
szURL := 'http://fs2.directupload.net/images/user/150607/x3sugvzy.jpg';
MAX_PATH := 512;
SetLength(szFileName, 513);
ErrorCode := URLDownloadToCacheFile(null, szURL, szFileName, MAX_PATH, 0, null);
MsgBox(szFileName,mbConfirmation, MB_OK);
end;

Answer 1

The original problem was that you were passing nil to a parameter of type Variant. Null value for the Variant type is NULL in Pascal Script.
But, your prototype needs to be changed. You need to use Unicode data types when you are using Unicode variant of a function, so you cannot use PAnsiChar types there. The next problem is with the Variant types you used for interface type parameters. That won't work. You can use IUnknown types there, because the lpUnkcaller parameter is of a pointer to this type and pBSC is of type IBindStatusCallback which from IUnknown inherits. The last problem is that MSDN is wrong here. The szURL is according to the header files of type LPCTSTR, not LPCSTR. The Unicode variant prototype looks like this in SDK 8.1 urlmon.h header file (formatted and commented by me):
STDAPI URLDownloadToCacheFileW(
_In_opt_ LPUNKNOWN,
_In_ LPCWSTR, // <- this is wrong on MSDN; this type would have to be LPCSTR here
_Out_writes_(cchFileName) LPWSTR,
DWORD cchFileName,
DWORD,
_In_opt_ LPBINDSTATUSCALLBACK
);
So, your script might be written like this (it should support also ANSI version of Inno Setup, but I haven't tested it there):
[Setup]
AppName=My Program
AppVersion=1.5
DefaultDirName={pf}\My Program
[Code]
const
MAX_PATH = 260;
S_OK = $00000000;
#ifdef UNICODE
#define AW "W"
#else
#define AW "A"
#endif
function URLDownloadToCacheFile(lpUnkcaller: IUnknown; szURL: string; szFileName: string;
cchFileName: DWORD; dwReserved: DWORD; pBSC: IUnknown): HRESULT;
external 'URLDownloadToCacheFile{#AW}#urlmon.dll stdcall';
function TryDownloadToCache(const URL: string; out FileName: string): Boolean;
begin
SetLength(FileName, MAX_PATH);
Result := URLDownloadToCacheFile(nil, URL, FileName, Length(FileName), 0, nil) = S_OK;
end;
function InitializeSetup: Boolean;
var
FileName: string;
ErrorCode: Integer;
begin
Result := True;
if TryDownloadToCache('http://i.imgur.com/wKCsei6.png', FileName) then
ShellExec('', FileName, '', '', SW_SHOW, ewNoWait, ErrorCode)
else
MsgBox('Downloading failed.', mbError, MB_OK);
end;