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Editorial Team
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Asked: 2026-06-01T10:20:51+00:00

I’ve written a DataSnap server method that returns a TStream object to transfer a

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I’ve written a DataSnap server method that returns a TStream object to transfer a file. The client application calls the method and reads the stream to download the file. The server method is very simple :

function TServerMethods.DownloadFile(sFilePath: string): TStream;
var
  strFileStream: TFileStream;
begin
  strFileStream := TFileStream.Create(sFilePath, fmOpenRead);
  Result := strFileStream;
end;

It works fine downloading many file types (PDF, GIF, BMP, ZIP, EXE) but it doesn’t work when downloading JPG files. On the client side the stream object returned from the method call is always 0 in size with JPGs. I can successfully stream JPG files locally on my PC, so it must be something to do with DataSnap. I’ve done some research which suggests DataSnap converts the stream to JSON behind the scenes and there could be a problem with this when it comes to JPG files – can anybody confirm this? On the client side I’m using the TDSRESTConnection to call the server method. I realise I could ZIP the JPG files before streaming, but would rather not have to do this.

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1 Answer

  1. Editorial Team

    Embarcadero have now come back with a fix to this problem (which also affects .DOC files) :

    1.Copy ‘…\RAD Studio\9.0\source\data\datasnap\Datasnap.DSClientRest.pas’ to your DataSnap Client project folder

    2.Add the .pas file to the project

    3.Modify Line#1288 as below

    //  LResponseJSON := TJSONObject.ParseJSONValue(BytesOf(LResponseText.StringValue), 0);
LResponseJSON := TJSONObject.ParseJSONValue(BytesOf(UTF8String(LResponseText.StringValue)), 0);

    4.Rebuild DataSnap REST Client project

    5.Run it with REST Server

    This fixes the problem.

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