Working with IStream in Delphi

At some point you might have to work with IStream. They are never fun to work with. Especially since you can't insert into a stream without overwriting. But that's streams.

I like to use the VCL wrappers, TOleStream and TStreamAdapter, and then use TMemoryStream. Use TOleStream to wrapping up an IStream making it look like a TStream and then use TIMemoryStream to make the TMemoryStream look like an IStream again.

Let's say for some reason we get a UTF-8 stream that needs to be saved as a file but it has no BOM. Here's a small example of how to add a BOM to the stream using Delphi:

uses ActiveX, AxCtrls, Classes;

function AddBomToStream(const Stream: IStream): IStream;
var
  Bom: TBytes;
  OleStream: TOleStream;
  MemoryStream: TMemoryStream;
  Temp: Largeint;
begin
  if Stream <> nil then
  begin
    MemoryStream := TMemoryStream.Create;
    Bom := TEncoding.UTF8.GetPreamble;
    MemoryStream.Write(Bom[0], Length(Bom));
    Stream.Seek(0, STREAM_SEEK_SET, Temp);
    OleStream := TOleStream.Create(Stream);

    try
      MemoryStream.CopyFrom(OleStream, OleStream.Size);
      Result := TStreamAdapter.Create(MemoryStream, soOwned) as IStream;
    finally
      OleStream.Free;
    end;
  end;
end;