Almost impossible to help without details on the .NET code you are using to connect and call Caché.

I think examples use very old version of Visual Studio

You are using an old version/product too, Caché 2018.1.

Congratulation Robert! 🏆🎖

Definitely deserved recognition.

The prerequisites for the InterSystems HL7 Interface Specialist certification include the training course and "At least 6 months - 1 year designing, building, and performing basic troubleshooting of HL7 interfaces with InterSystems products version 2019.1 or higher."

Actual/real experience will definitely help in passing the certification, theoretical study and some exercise may not be enough to gain the knowledge required.

In the exam description page you can find a list of "Exam Topics and Content". All questions in the exam belong to those topics.

Last but not least, everything is contained in the documentation.

Where/when do you want to know it?

Usually you get this information looking at the trace of the session/message.

Or you need to know the sender from within your BO code/method?

I don't think there is a callback method available.

One option could be to subclass %CSP.REST and use your subclass instead of %CSP.REST.

You subclass can be something like:

Class MyCustomCSP.REST Extends %CSP.REST
{

ClassMethod DispatchRequest(url As %String, method As %String, forwarded As %Boolean = 0, args...) As %Status
{
	Set sc=##super(url,method,forwarded,.args)
	
	; your "onPostDispatch" code here 
	Quit sc
}

}

Just in case, try adding a "code" action BEFORE your actions with:

  Do source.Stream.Rewind()
  Do target.Stream.Clear()

How big (Size) is the stream? If it's larger then the maximum string supported string length (~3.5MB), then you have a problem with your code.

When it does not work, do you get an error?

In the DTL, how do you set the "Create" option/parameter? (copy or new?)

Is there any other reference/set referencing the Stream in the DTL?

What product? If it's HealthShare then you should use the class HS.Local.ZAUTHENTICATE in HSCUSTOM namespace.

In fact, for simple XML export (no namespace and other amenities) you can just use XMLExport*() methods.

ClassMethod test()
{
	Set message=##class(Mensajes.Response.Radiologia.NumeroOrdenAcodigoSERAMResponse).%New()
	Set message.resultado.codigo="06050301"
	Set message.resultado.error=##class(EsquemasDatos.Seguridad.Error).%New()
	Set message.resultado.error.codigo=0
	Set message.resultado.error.descripcion="Proceso realizado correctamente"
	
	Set sc=message.resultado.XMLExportToString(.string)
    ; handle sc error here

	Write string,!!
	
	; convert XML string back to object
    Set reader = ##class(%XML.Reader).%New()
    Set sc=reader.OpenString(string)
    ; handle sc error here
    Do reader.Correlate("resultado","EsquemasDatos.Radiologia.Resultado")
    Do reader.Next(.ReturnObject,.sc)
    ; handle sc error here
    Write "ReturnObject is of type ",ReturnObject.%ClassName(1),!
    do ReturnObject.XMLExport(,",indent")
	
	Quit
}

Result:

Do ##class(Mensajes.Response.Radiologia.NumeroOrdenAcodigoSERAMResponse).test()
<resultado><error><codigo>0</codigo><descripcion>Proceso realizado correctamente</descripcion></error><codigo>06050301</codigo></resultado>
 
ReturnObject is of type EsquemasDatos.Radiologia.Resultado
<resultado>
  <error>
    <codigo>0</codigo>
    <descripcion>Proceso realizado correctamente</descripcion>
  </error>
  <codigo>06050301</codigo>
</resultado>

OK, you asked XML string to object 😊

ClassMethod test()
{
	Set message=##class(Mensajes.Response.Radiologia.NumeroOrdenAcodigoSERAMResponse).%New()
	Set message.resultado.codigo="06050301"
	Set message.resultado.error=##class(EsquemasDatos.Seguridad.Error).%New()
	Set message.resultado.error.codigo=0
	Set message.resultado.error.descripcion="Proceso realizado correctamente"
	
	Set writer=##class(%XML.Writer).%New()
	Set writer.Indent=1

	Set sc=writer.OutputToString()
	; handle sc error here
	Set sc=writer.RootObject(message.resultado)
	; handle sc error here
	Set string=writer.GetXMLString()
	Write string,!
	
	; convert XML string back to object
    Set reader = ##class(%XML.Reader).%New()
    Set sc=reader.OpenString(string)
    ; handle sc error here
    Do reader.Correlate("resultado","EsquemasDatos.Radiologia.Resultado")
    Do reader.Next(.ReturnObject,.sc)
    ; handle sc error here
    Write "ReturnObject is of type ",ReturnObject.%ClassName(1),!
    do ReturnObject.XMLExport(,",indent")
	
	Quit
}

Result:

Do ##class(Mensajes.Response.Radiologia.NumeroOrdenAcodigoSERAMResponse).test()
<resultado>
  <error>
    <codigo>0</codigo>
    <descripcion>Proceso realizado correctamente</descripcion>
  </error>
  <codigo>06050301</codigo>
</resultado>
 
ReturnObject is of type EsquemasDatos.Radiologia.Resultado
<resultado>
  <error>
    <codigo>0</codigo>
    <descripcion>Proceso realizado correctamente</descripcion>
  </error>
  <codigo>06050301</codigo>
</resultado>

To export a XML enabled class use %XML.Writer class.

ClassMethod test()
{
	Set message=##class(Mensajes.Response.Radiologia.NumeroOrdenAcodigoSERAMResponse).%New()
	Set message.resultado.codigo="06050301"
	Set message.resultado.error=##class(EsquemasDatos.Seguridad.Error).%New()
	Set message.resultado.error.codigo=0
	Set message.resultado.error.descripcion="Proceso realizado correctamente"
	
	Set writer=##class(%XML.Writer).%New()
	Set writer.Indent=1

	Set sc=writer.OutputToString()
	; handle sc error here
	Set sc=writer.RootObject(message.resultado)
	; handle sc error here
	Set string=writer.GetXMLString()
	Write string,!
	Quit $$$OK
}

In your EsquemasDatos.Radiologia.Resultado class add XMLNAME parameter to obtain the correct (lowercase) tag name:

Class EsquemasDatos.Radiologia.Resultado Extends (%SerialObject, %XML.Adaptor)
{

Parameter XMLNAME = "resultado";

Property error As EsquemasDatos.Seguridad.Error;

Property codigo As %String(MAXLEN = "");

Result:

Do ##class(Mensajes.Response.Radiologia.NumeroOrdenAcodigoSERAMResponse).test()
<resultado>
  <error>
    <codigo>0</codigo>
    <descripcion>Proceso realizado correctamente</descripcion>
  </error>
  <codigo>06050301</codigo>
</resultado>

In your OnProcessInput() methods, how do you call the business process (or operation)? SendRequestAsync() or SendRequestSync()?

How can a process commit/rollback transactions of other process(es)??

Are you using "direct" file I/O using Open/Use etc. commands?
I think that's the way to go for your requirements.

If so, have a look to the $ZSEEK() function, the documentation is here.