Save a file downloaded via WinHTTP to disk, using Delphi XE

Question

An answer to this question showed how easy it is to use WinHTTP via Type Library imports in delphi.

I imported the type library for WinHTTP, and then tried to write a File Download helper function using that api. Here's how far I got:

I can't seem to figure out how to save the IWinHttpRequest.ResponseStream (declared as OleVariant in the TLB file) as Stream, to disk.

// IWinHttpRequest is defined by importing type library of WinHTTP.
// Microsoft WinHTTP Services, version 5.1 (Version 5.1) C:\Windows\system32\winhttp.dll
function Download(const url, filename: String): Boolean;
var
   http: IWinHttpRequest;
   wUrl: WideString;
   fs:TFileStream;
   FileStream:IStream;
   sz,rd,wr:Int64;
begin
  try
   wUrl := url;
   http := CoWinHttpRequest.Create;
   http.open('GET', wurl, False);
   http.send(EmptyParam);

   FStatus := http.status; // 200=OK!
   result := FStatus=200;


   if result then
   begin
     fs := TFileStream.Create(filename, fmCreate, fmShareExclusive );
     try
      FileStream := TStreamAdapter.Create(fs, soReference) as IStream;
      sz := http.ResponseStream.Size;
      http.ResponseStream.CopyTo(FileStream,sz,rd,wr);
     finally
         FileStream :=  nil;
         fs.Free;
     end;
   end;
  except
      result := false;
      // do not raise exceptions.
  end;
end;

Excerpt from WinHTTP_TLB.pas:

 IWinHttpRequest = interface(IDispatch)
    ['{016FE2EC-B2C8-45F8-B23B-39E53A75396B}']
    ......
    property ResponseStream: OleVariant read Get_ResponseStream;

Update: I now get a runtime exception about ole variants, at the call to http.ResponseStream.CopyTo(...)

 EOleError 'Variant does not reference an automation object'.

related: http://stackoverflow.com/questions/2771551/cast-object-as-olevariant-in-delphi — Warren P, Jul 22 '11 at 21:02
TOleStream related: http://stackoverflow.com/questions/4443376/how-to-free-tolestream-in-this-bit-of-code — Warren P, Jul 23 '11 at 14:48

score 6 · Accepted Answer · answered Jul 22 '11 at 21:00

Warren, you must use the AxCtrls.TOleStream class to comunicate the response stream with the Classes.TFileStream

something like this

IWinHttpRequest.ResponseStream -> TOleStream -> TFileStream

Check this sample code

{$APPTYPE CONSOLE}

uses
  Variants,
  ActiveX,
  Classes,
  AxCtrls,
  WinHttp_TLB,
  SysUtils;


function Download(const url, filename: String): Boolean;
var
   http: IWinHttpRequest;
   wUrl: WideString;
   fs:TFileStream;
   HttpStream :IStream;
   sz,rd,wr:Int64;
   FStatus : Integer;
   OleStream: TOleStream;
begin
  try
   wUrl := url;
   http := CoWinHttpRequest.Create;
   http.open('GET', wurl, False);
   http.send(EmptyParam);

   FStatus := http.status; // 200=OK!
   result := FStatus=200;

   if result then
   begin
    HttpStream:=IUnknown(http.ResponseStream) as IStream;
    OleStream:= TOleStream.Create(HttpStream);
    try
      fs:= TFileStream.Create(FileName, fmCreate);
      try
        OleStream.Position:= 0;
        fs.CopyFrom(OleStream, OleStream.Size);
      finally
        fs.Free;
      end;
    finally
      OleStream.Free;
    end;
   end;

  except
      result := false;
      // do not raise exceptions.
  end;
end;


begin
  try
    Download('http://foo.html','C:\Foo\anyfile.foo');
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
end.

Save a file downloaded via WinHTTP to disk, using Delphi XE

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