Apparent random publisher behaviour

Thanks for the suggestion, but I'm not sure if I get the idea, I'm still begining with all this. I've been searching for examples of use of on_subscription_matched but I only see prints when they're called. But if I understood it well:

1) we have a DataReaderListener that creates a data reader with the topic name "my_name"

2) a publisher out there creates a data writer with a topic having the name "my_name" too

3) the on_subscription_matched from the first DataReaderListener is triggered

But what I do is the following:

1) the server creates a data reader with a topic of name "client-to-server requests"

2) client (all clients) creates a data writer with a topic of name "client-to-server requests" too

3) user submits the login data

4) the client creates a data reader with a topic of name username +" topic"

5) the client sends via the "client-to-server requests" the login data to the server

6) server receives that message and uses the username to create a data writer with a topic of name username +" topic"

7) the server sends the login request reply using that data writer

Then I don't understand which are those matches when you say "so that the client doesn't send their message before having x matches " in this scenario.

About the QoS, everything is in x_QOS_DEFAULT.

Thanks for putting me on the right track. This is what I did, in case is useful to someone:

public class MyClassPublisher : DDS.DataWriterListener {

...

while (!this.discovered) { Thread.Sleep(500); }
myDataWriter.write(msg, ref DDS.InstanceHandle_t.HANDLE_NIL);

...

/* This callback is called when the DDS::DataWriter has found a DDS::DataReader that matches the DDS::Topic, has a common partition and compatible QoS, 
* or has ceased to be matched with a DDS::DataReader that was previously considered to be matched.*/
public override void on_publication_matched(DDS.DataWriter writer, ref DDS.PublicationMatchedStatus status) {
      this.discovered = true;
}

Ok. False positive. Still same problem: A sends to B, the message reaches to B, B replies but A only sees the answer 'sometimes'. I'm sending just a few messages, but these are used to communicate orders, so losing one of them could be a problem.

About on_publication_matched the reference says: "This callback is called when the DDS::DataWriter has found a DDS::DataReader that matches the DDS::Topic, has a common partition and compatible QoS, or has ceased to be matched with a DDS::DataReader that was previously considered to be matched." I was not sure if this method was called every time you write or just the first time you write with a particular data writer. With a print I see that it seems to be called just the first time.

Then, to check if there is at least one reader listening a particular writer, looking as you suggested to current_count > 1, I want to do something like this.

public override void on_publication_matched(DDS.DataWriter writer, ref DDS.PublicationMatchedStatus status) {
      System.Diagnostics.Debug.WriteLine("Publisher - on_publication_matched " + writer.get_topic().???? + " status.current_count:" + status.current_count);
      this.discovered = true;
}

The thing now is that looking at the reference I can't find the way to get the name of the topic of that writer to distinguish which of them saw a reader and hence can already send messages. 

 Edit: New attempt. Seems to work better: 50 messages sent, just one message lost.

public CommandMsgDataWriter writer_discovered;

public override void on_publication_matched(DDS.DataWriter writer, ref DDS.PublicationMatchedStatus status) {
     this.writer_discovered = (CommandMsgDataWriter)writer;
}

public void write(CommandMsg c) {
    while (this.writer_discovered != datawriters[c.pilot]) { Thread.Sleep(1); }
     datawriters[c.pilot].write(c, ref DDS.InstanceHandle_t.HANDLE_NIL);
}