How to send and receive a TMemoryStream (in a TObject) using DataSnap in Delphi 10.1

Question

I have an application written in Delphi 10.1 using REST Datasnap.

This application includes a Client and Server.

The Client is a mobile application (Android) and the Server is a Windows Service that's is connected to a firebird database.

I have an Object:-

TJob = class(TObject)
  private
    FID: Integer;
    FThe_Name: String;
    FImage: TMemoryStream;
  public
    constructor Create;
    destructor Destroy;
  end;

constructor TJob.Create;
begin
  inherited;
  FImage := TMemoryStream.Create;
end;

destructor TJob.Destroy;
begin
  FreeAndNil(FImage);
  inherited;
end;

I get an access violation when I try to save the image on the server to the DB and when I try and open and display the image on the client.

I have a standalone multidevice application that use the same functionality(Getting, Saving and displaying of an Image) as the Client/Server and works.

Client displaying the image on the form:-

if (Job.Image.Size > 0) then
  begin
    rectangle.Fill.Kind := TBrushKind.Bitmap;
    rectangle.Fill.Bitmap.Bitmap.LoadFromStream(Job.Image);
    rectangle.Repaint;
    Layout.Repaint;
  end;

Client getting the Image from the form:-

if not(rectangle.Fill.Bitmap.Bitmap.IsEmpty) then
  begin
    Job.Image.Seek(0, soFromBeginning);
    rectangle.Fill.Bitmap.Bitmap.SaveToStream(Job.Image);
    Job.Image.Position := 0;
  end;

Server Saving the Image to DB:-

  Job.Image.Position := 0;
  (TBlobField(FieldByName('MyImage'))).SaveToStream(Job.Image);

Server getting the Image from DB:-

(TBlobField(FieldByName('MyImage'))).SaveToStream(Job.Image);

The Standalone application works using the same however I get errors when trying to either save or display an image.

I have populated the DB with various formats of images, which I can view in the DB, but not from the Client(AV).

Any ideas on what I've done wrong and examples on how to solve fix?

Thanks

There isn't enough code here. We will need to see the complete client code that calls the server method, and the server method code. — Freddie Bell, Jun 15 '20 at 13:59
This may or may not be a red herring but as I'm passing a TMemorystream does it need to involve Marshalling? I have no idea how that works so examples would be grateful. — Yuppski, Jun 15 '20 at 17:51
Is the image you are using bigger than 256kb ?, because I had problems passing streams bigger than that, they get truncated, so I had to pass them split in chunks. https://stackoverflow.com/questions/41854631/cant-retrieve-tstreams-bigger-than-around-260-000-bytes-from-a-datasnap-server — Marc Guillot, Jun 16 '20 at 06:08
Thanks for the pointers Marc. I found and used Base64FromBitmap & BitmapFromBase64. I've updated the main post with my latest source. 1. My client will only handle BMP files (Just to add a standlone app works fine with most types). 2. My Server has an AV when trying to save, calling the BitmapFromBase64(This does not occur on my standlone) — Yuppski, Jun 16 '20 at 14:16

Yuppski · Accepted Answer · 2020-06-22T13:07:01.080

Server getting the Image from DB:-

  PngImage := TPngImage.Create;
  MemoryStream := TMemoryStream.Create;
  try
    (TBlobField(FieldByName('Image'))).SaveToStream(MemoryStream);
    MemoryStream.Position := 0;
    PngImage.LoadFromStream(MemoryStream);
    Job.Image_AsStr := Base64FromPngImage(PngImage);
  finally
    MemoryStream.Free;
  end;

Client displaying the image on the form:-

  if (Job.Image_AsStr <> '') then
  begin
    rImage.Fill.Kind := TBrushKind.Bitmap;  
    rImage.Fill.Bitmap.Bitmap := BitmapFromBase64(Job.Image_AsStr);
    rCustomer_Signature.Repaint;
    lCustomer_Signature.Repaint;
  end;

Client getting the Image from the form:-

  if not(rImage.Fill.Bitmap.Bitmap.IsEmpty) then
  begin
    rImage.Fill.Kind := TBrushKind.Bitmap;
    Job.Image_AsStr := Base64FromBitmap(rImage.Fill.Bitmap.Bitmap);
  end
  else
    Job.Image_AsStr := '';

Server Saving the Image to DB:-

if Job.Image_AsStr <> '' then
begin
  MemoryStream := TMemoryStream.Create;
  try
    PngImage := PngImageFromBase64(Job.Image_AsStr);
    PngImage.SaveToStream(MemoryStream);
    MemoryStream.Position := 0;
    Params[1].LoadFromStream(MemoryStream, ftBlob);
  finally
    MemoryStream.Free;
  end;
end
else
  Params[1].Clear;

On my experience the image arrives back at the Server as a PngImage even though its been packaged as a BMP.

I can now confirm that I've installed and tested on a Android phone. *Note. The original question posted was about using a TMemoryStream, these examples uses a String.

How to send and receive a TMemoryStream (in a TObject) using DataSnap in Delphi 10.1

1 Answers1