How to pass and return a TStream using DataSnap in C++ Builder

Question

I have seen lots of DataSnap examples in Delphi, but fewer in C++ Builder, and have not figured out how to specify that a TStream should be returned to the calling client.

I am using a simple configuration, similar to the tutorials I have seen. An example server method is:

    System::UnicodeString GetData(int PatientID, int& count, TStream* stream);

I have no trouble calling that method from my client. Because count is passed as a reference, the DataSnap server knows to send it back to the client. Generate Client Classes, on the TSQLConnection in the client, connects to the server, and generates the following:

System::UnicodeString __fastcall TServerMethods1Client::GetData(int PatientID, int &count, TStream* stream)
{
  if (FGetDataCommand == NULL)
  {
    FGetDataCommand = FDBXConnection->CreateCommand();
    FGetDataCommand->CommandType = TDBXCommandTypes_DSServerMethod;
    FGetDataCommand->Text = "TServerMethods1.GetData";
    FGetDataCommand->Prepare();
  }
  FGetDataCommand->Parameters->Parameter[0]->Value->SetInt32(PatientID);
  FGetDataCommand->Parameters->Parameter[1]->Value->SetInt32(count);
  FGetDataCommand->Parameters->Parameter[2]->Value->SetStream(stream, FInstanceOwner);
  FGetDataCommand->ExecuteUpdate();
  count = FGetDataCommand->Parameters->Parameter[1]->Value->GetInt32();
  System::UnicodeString result = FGetDataCommand->Parameters->Parameter[3]->Value->GetWideString();
  return result;
}

One can see that the generated code is setting the count from the returned parameter, indicating that the server is sending it back. However, the stream is only sent to the server, and not set back on the client.

In Delphi, I would use var to indicate that the reference should be passed back to the caller. However, using a reference on TStream does not work, either.

For this definition:

    System::UnicodeString GetData(int PatientID, int& count, TStream& stream);

I get this generated code:

System::UnicodeString __fastcall TServerMethods1Client::GetData(int PatientID, int &count, TStream* &stream)
{
  if (FGetDataCommand == NULL)
  {
    FGetDataCommand = FDBXConnection->CreateCommand();
    FGetDataCommand->CommandType = TDBXCommandTypes_DSServerMethod;
    FGetDataCommand->Text = "TServerMethods1.GetData";
    FGetDataCommand->Prepare();
  }
  FGetDataCommand->Parameters->Parameter[0]->Value->SetInt32(PatientID);
  FGetDataCommand->Parameters->Parameter[1]->Value->SetInt32(count);
  FGetDataCommand->Parameters->Parameter[2]->Value->SetStream(stream, FInstanceOwner);
  FGetDataCommand->ExecuteUpdate();
  count = FGetDataCommand->Parameters->Parameter[1]->Value->GetInt32();
  stream = FGetDataCommand->Parameters->Parameter[2]->Value->GetStream(FInstanceOwner);
  System::UnicodeString result = FGetDataCommand->Parameters->Parameter[3]->Value->GetWideString();
  return result;
}

Which throws an access violation in the ExecuteUpdate() call.

Is there a way I can pass a pointer to the server method and mark it in some way that the stream should be passed back to the calling client?

Answer 1

Remy's response was correct.

The server method is

System::UnicodeString TServerMethods1::GetData(int PatientID, int& count, 
    TStream*& stream) {
count = 10;
String newSimpleString = "New Simple String";
TByteDynArray theBytes;
theBytes.Length = newSimpleString.Length();
for (int i = 0; i < newSimpleString.Length(); i++) {
    theBytes[i] = newSimpleString[i + 1];
}

TDBXBytesStream* newStream =
    new TDBXBytesStream(theBytes, newSimpleString.Length());
stream = newStream;
return "StringResult";

}

and the client method is

void __fastcall TForm1::Button2Click(TObject *Sender) {
int count = 1;
TStringStream* stringStream = new TStringStream(String("Passed In"));
TStream* str = stringStream;
String result = ClientModule1->ServerMethods1Client->GetData(1, count, str);
int len = str->Size;
ShowMessage("Result is: '" + result + "' and " + String(count) +
    ", and stream is " + String(len) + " bytes long");

}

With FInstanceOwner at true (the default as generated), the stream passed in as a client method gets freed the next time the method is called.

On the server, the newly created TDBXBytesStream gets freed the next time the server method is called.

TDBXBytesStream is not a fully functional stream - for example, I could not get CopyFrom to copy from a TStringStream (for example), which is why I use the TByteDynArray.

Also, I could not modify the stream passed in to the server method by writing to it, which is why I needed to assign a new TDBXBytesStream to the passed in reference.

Thanks, Remy, for helping me with this.