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C# Remoting Register a class from Host App as service

I am developing a solution to run on local network.
I have successfully created a dummy solution that hosts a assembly dll as a service (using Remoting) in a WinForms application and listens to the client communication through proxy object.
Now I need to extend the functionality of the service so that it can perform more complex operations which will be part of the Host App, for this reason I tried registering service from a local class and this is where I hit a dead-end, it is throwing ArgumentNullException.
I have two questions:
Is it even possible to register a class from an executable app as a service? If yes, what is the correct procedure?
If no, then how can the assembly dll further notify the Host App of the data recieved?
P.S : I am new to C#. Please excuse any stupid mistakes.
EDIT:
The requirement is for one way communication only. Would WCF still be the best way to proceed?

can't find main static main method in WCF

I created WCF service and faced with a problem. I need to update database periodically, but i couldn't find static method like Main, that whould do it without client interaction. What can i do??? What wold you suggest in such case?

There is no Main method (or similar entry point) in WCF. You need to host your WCF service in another process (such as a Windows service, or IIS or self host) to "activate" it and make it available to other processes.
One of the concepts in WCF is that you write your service code to do the function you need without having to worry about infrastructure and hosting. Once you have written your service logic, you can then decorate and configure your service to expose it to other processes. Using this approach means you can change how your service is exposed to other processes without re-writing the actual service logic - you essentially just change your configuration. Hence, a main entry point is specific to how you choose to host and expose your WCF service to the outside world.
Just Google around for "WCF hosting" and you will find lots of information.
If you don't need to expose your service logic to an external process (which sounds like maybe the case from your question) then maybe you don't need to use WCF and you can just write a plain old Windows Service.

If your wcf service is self hosted then you can do it in your application before publishing the service.
If it is in IIS then there really isn't application_start kind of thing since the host may be created on first request. See WCF application start event

Consuming the same WCF service from different addresses

I have a single WCF service but, until runtime, I don't know the correct address of the service. It may be :
http://example1.com/MyService.svc
// or
http://example2.com/MyService.svc
The service is used by a class library (DAL). I have two options:
Add a service reference to the
service (Visula Studio 2010) and
change the address at run-time. This
way VS-2010 will create WSDL and
other stuff for me (I'm not sure if this is even possible).
Create the proxy on the fly and set
the base service address. This needs
more work and if I make any change
to service, I need to generate WSDL
myself. Maintenance of this code is
not as easy as option one.
Which option to use? Also if option two is recommended by you, then should I my client wrapper class be singleton or I can create all the connection stuff on each call?

you can point to localhost or any other address in development then in production if the url changes you simply modify the web.config or the app.config where you have configured the WCF end point.

Option 1 - you get all the advantages and none of the pain. Just use something factory-oriented (i.e. Don't do new MyProxy(), but instead stick that code somewhere central like a static CreateMyProxy() method, or consider an IoC/DI container).
How to consume WCF web service through URL at run time?

WCF services and firewalls... Any issues?

Simple issue. I'm working on a proof-of-concept for an application with additional database connection, so I will create a WCF service to wrap around the database. Multi-user environments will get this service installed on a centralized server with a client application on their local system. These users will automatically have to deal with firewall issues, so this is acceptable.
But the single-user environments will have the service and client application running on a single system. The service host doesn't have a definite shape right now, but it's likely that it will be hosted within the application itself or as a Windows service.
Unfortunately, the client application is a WIN32 Delphi application which needs a simple way to access the service. Preferably, the single-user version should use the same technique to access the server as the multi-user version, which means that it behaves like a SOAP client, with a WSDL imported and converted to Delphi code.
Still not a problem, but I have to consider possible problems that we can encounter in this setup, with the most important issue: possible firewall that closed the connection port.
So, does anyone know about any firewall problems that can occur in this single-user environment?

You haven't mentioned which WCF channel you're using- I'll assume basicHttpBinding. Generally, if your local service is bound to 127.0.0.1 using self-hosting, and the on-box client accesses it that way, you should be fine. No firewalls I'm aware of will screw with your loopback adapter. If you bind the service to the machine's IP though, you may subject yourself to firewall fun.
If you have WCF 3.5 available on the client on both ends (sorry, I don't know anything about Delphi), have a go with netNamedPipeBinding.

You didn't mention which version of Delphi you use but I once had hard time making Delphi 2005 import a WCF service with basicHttpBinding. As the WSDL is split among many pages, the SOAP import wizard in Delphi wasn't capable of understanding it. I finally ended up writing an ASMX wrapper around the WCF service for Delphi clients.

How to store WCF sessions so another application can access them

Hi I have an application that operations like this..
Client <----> Server <----> Monitor Web Site
WCF is used for the communication and each client has its own session on the server. This is so callbacks can be used from the server to callback to the client.
The objective is that a user on the "Monitor Website" can do the following:
a) Look at all of the users currently online - that is using the client application.
b) Select a client and then perform an action on the client.
This is a training system so the idea being the instructor using a web terminal can select his or her target client and then make the client application do something. Or maybe they want to send a message to the client that will be displayed on the clients screen.
What I cant seem to do is to store a list of all the clients in the server application, that can then be retrieved by the server. If I could do this I could then access the callback object for the client and call the appropriate method.
A method on the monitoring website would look something like this...
Service.SendMessage(userhashcode, message)
The service would then somehow look up the callback that matches the hashcode and then do something like this
callback.SendMessage(message)
So far I have tried without look to serialise the callbacks into a centralised DB. However, it doesnt seem possible on the service to serialise a remote object as the callback exists from the client.
Additionally I thought I could create a global hash table in my service but im not sure on how to do this and to make it accesible application wide.
Any help would be appreciated.

Typically, WCF services are "per-call" only, e.g. each caller gets a fresh instance of the service class, it handles the request, formats the response, send it back and then gets disposed. So typically, you don't have anything "session-like" hanging around in memory.
What you do have is not the service classes themselves, but the service host - the class that acts as the host for your service classes. This is either IIS (in that case you just need to monitor IIS), or then it's a custom app (Windows NT Service, console app) that has a ServiceHost instance up and running.
I am not aware what kind of hooks there might be to connect to and "look inside" the service host - but that's what you're really looking for, I guess.
WCF services can also be configured to be session-ful, and keep a session up and running with a service class - but again: you need to have that turned on explicitly. Even then, I'm not really sure if you have many API hooks to get "inside" the service host and have a look around the current sesssions.
Question is: do you really need to? WCF exposes a gazillion of performance counters, so you can monitor and record just about anything that goes on in WCF - wouldn't that be good enough for you?
Right now, WCF services aren't really hosted in a particularly well-designed system - this should become better with the so-called "Dublin" server-addon, which is designed to host WCF services and WF workflows and give admins a great experience monitoring and managing them. "Dublin" is scheduled to be launched shortly after .NET 4.0 becomes available (which Microsoft has promised will be before the end of calendar year 2009).
Marc

What I have done is as follows...
Created a static instance in my service that keeps a dictionary of callbacks keyed by the hashcode of each WCF connection.
When a session is created it publishes itself to a DB table which contains the hash code and additional connection information.
When a user is using the monitor web application, it can get a list of connected clients from the DB and get the hashcode for that client.
If the monitor application user wants to send a command to the client the following happens..
The hashcode for the sessionn is obtained from the db.
A method is called on the service e.g. SendTextMessage(int hashcode, string message).
This method now looks up the callback to the client from the dictionary of callbacks and obtains a reference to it.
The appropriate method in this case SendTextMessage(message) is called on the callback.
Ive tested this and it works ok, Ive also added a functionality to keep the DB table synchronised to the actual WCF sessions and to clean up as required.