3.6 Writing an Ice Application with Visual Basic

This section shows how to create an Ice application with Visual Basic.

Overview

As of version 3.3, Ice no longer includes a separate compiler to create Visual Basic source code from Slice definitions. Instead, you need to use the Slice-to-C# compiler slice2cs to create C# source code and compile the generated C# source code with a C# compiler into a DLL that contains the compiled generated code for your Slice definitions. Your Visual Basic application then links with this DLL and the Ice-for-.NET DLL (Ice.dll).¹

Figure 3.1 illustrates this development process.

Figure 3.1. Developing a Visual Basic application with Ice.

Compiling a Slice Definition for Visual Basic

The first step in creating our VB application is to compile our Slice definition to generate proxies and skeletons. You can compile the definition as follows:

$ mkdir generated
$ slice2cs ‑‑output‑dir generated Printer.ice

The ‑‑output‑dir option instructs the compiler to place the generated files into the generated directory. This avoids cluttering the working directory with the generated files. The slice2cs compiler produces a single source file, Printer.cs, from this definition. The exact contents of this file do not concern us for now—it contains the generated code that corresponds to the Printer interface we defined in Printer.ice.

We now need to compile this generated code into a DLL:

$ csc /reference:Ice.dll /lib:%ICE_HOME%\bin /t:library /out:Printer.dll generated\Printer.cs

This creates a DLL called Printer.dll that contains the code we generated from the Slice definitions.

Writing and Compiling a Server

To implement our Printer interface, we must create a servant class. By convention, servant classes use the name of their interface with an I‑suffix, so our servant class is called PrinterI and placed into a source file Server.vb:

Imports System
Imports Demo

Public Class PrinterI
    Inherits PrinterDisp_

    Public Overloads Overrides Sub printString( _
                                ByVal s As String, _
                                ByVal current As Ice.Current)
        Console.WriteLine(s)
    End Sub

End Class

The PrinterI class inherits from a base class called _PrinterDisp, which is generated by the slice2cs compiler. The base class is abstract and contains a printString method that accepts a string for the printer to print and a parameter of type Ice.Current. (For now we will ignore the Ice.Current parameter. We will see its purpose in detail in Section 28.6.) Our implementation of the printString method simply writes its argument to the terminal.

The remainder of the server code follows in Server.vb and is shown in full here:

Module Server

    Public Sub Main(ByVal args() As String)

        Dim status As Integer = 0
        Dim ic As Ice.Communicator = Nothing
        Try
            ic = Ice.Util.initialize(args)
            Dim adapter As Ice.ObjectAdapter = _
                ic.createObjectAdapterWithEndpoints( _
                    "SimplePrinterAdapter", "default ‑p 10000")
            Dim obj As Ice.Object = New PrinterI
            adapter.add(obj, ic.stringToIdentity( _
                                        "SimplePrinter"))
            adapter.activate()
            ic.waitForShutdown()
        Catch e As Exception
            Console.Error.WriteLine(e)
            status = 1
        End Try
        If Not ic Is Nothing Then
            ' Clean up
            '
            Try
                ic.destroy()
            Catch e As Exception
                Console.Error.WriteLine(e)
                status = 1
            End Try
        End If
        Environment.Exit(status)
    End Sub

End module

Note the general structure of the code:

Module Server

    Public Sub Main(ByVal args() As String)

        Dim status As Integer = 0
        Dim ic As Ice.Communicator = Nothing
        Try

            ' Server implementation here...

        Catch e As Exception
            Console.Error.WriteLine(e)
            status = 1
        End Try
        If Not ic Is Nothing Then
            ' Clean up
            '
            Try
                ic.destroy()
            Catch e As Exception
                Console.Error.WriteLine(e)
                status = 1
            End Try
        End If
        Environment.Exit(status)
    End Sub

End module

The body of Main contains a Try block in which we place all the server code, followed by a Catch block. The catch block catches all exceptions that may be thrown by the code; the intent is that, if the code encounters an unexpected run-time exception anywhere, the stack is unwound all the way back to Main, which prints the exception and then returns failure to the operating system.

Before the code exits, it destroys the communicator (if one was created successfully). Doing this is essential in order to correctly finalize the Ice run time: the program must call destroy on any communicator it has created; otherwise, undefined behavior results.

The body of our Try block contains the actual server code:

            ic = Ice.Util.initialize(args)
            Dim adapter As Ice.ObjectAdapter = _
                ic.createObjectAdapterWithEndpoints( _
                    "SimplePrinterAdapter", "default ‑p 10000")
            Dim obj As Ice.Object = New PrinterI
            adapter.add(obj, ic.stringToIdentity( _
                                        "SimplePrinter"))
            adapter.activate()
            ic.waitForShutdown()

The code goes through the following steps:

1. We initialize the Ice run time by calling Ice.Util.initialize. (We pass args to this call because the server may have command-line arguments that are of interest to the run time; for this example, the server does not require any command-line arguments.) The call to initialize returns an Ice::Communicator reference, which is the main handle to the Ice run time.

2. We create an object adapter by calling createObjectAdapterWithEndpoints on the Communicator instance. The arguments we pass are "SimplePrinterAdapter" (which is the name of the adapter) and "default ‑p 10000", which instructs the adapter to listen for incoming requests using the default protocol (TCP/IP) at port number 10000.

3. At this point, the server-side run time is initialized and we create a servant for our Printer interface by instantiating a PrinterI object.

4. We inform the object adapter of the presence of a new servant by calling add on the adapter; the arguments to add are the servant we have just instantiated, plus an identifier. In this case, the string "SimplePrinter" is the name of the servant. (If we had multiple printers, each would have a different name or, more correctly, a different object identity.)

5. Next, we activate the adapter by calling its activate method. (The adapter is initially created in a holding state; this is useful if we have many servants that share the same adapter and do not want requests to be processed until after all the servants have been instantiated.)

6. Finally, we call waitForShutdown. This call suspends the calling thread until the server implementation terminates, either by making a call to shut down the run time, or in response to a signal. (For now, we will simply interrupt the server on the command line when we no longer need it.)

Note that, even though there is quite a bit of code here, that code is essentially the same for all servers. You can put that code into a helper class and, thereafter, will not have to bother with it again. (Ice ships with such a helper class, called Ice.Application—see Section 16.3.1.) As far as actual application code is concerned, the server contains only a few lines: ten lines for the definition of the PrinterI class, plus three² lines to instantiate a PrinterI object and register it with the object adapter.

We can compile the server code as follows:

$ vbc /reference:Ice.dll /libpath:%ICE_HOME%\bin /reference:Printer.dll /out:server.exe Server.vb

This compiles our application code and links it with the Ice-for-.NET run time and the DLL we generated earlier. We assume that the ICE_HOME environment variable is set to the top-level directory containing the Ice run time. (For example, if you have installed Ice in C:\opt\Ice, set ICE_HOME to that path.)

Writing and Compiling a Client

The client code, in Client.vb, looks very similar to the server. Here it is in full:

Imports System
Imports Demo

Module Client

    Public Sub Main(ByVal args() As String)
        Dim status As Integer = 0
        Dim ic As Ice.Communicator = Nothing
        Try
            ic = Ice.Util.initialize(args)
            Dim obj As Ice.ObjectPrx = ic.stringToProxy( _
                                "SimplePrinter:default ‑p 10000")
            Dim printer As PrinterPrx = _
                                PrinterPrxHelper.checkedCast(obj)
            If printer Is Nothing Then
                Throw New ApplicationException("Invalid proxy")
            End If

            printer.printString("Hello World!")
        Catch e As Exception
            Console.Error.WriteLine(e)
            status = 1
        End Try
        If Not ic Is Nothing Then
            ' Clean up
            '
            Try
                ic.destroy()
            Catch e As Exception
                Console.Error.WriteLine(e)
                status = 1
            End Try
        End If
        Environment.Exit(status)
    End Sub

End Module

Note that the overall code layout is the same as for the server: we use the same Try and Catch blocks to deal with errors. The code in the Try block does the following:

1. As for the server, we initialize the Ice run time by calling Ice.Util.initialize.

2. The next step is to obtain a proxy for the remote printer. We create a proxy by calling stringToProxy on the communicator, with the string "SimplePrinter:default ‑p 10000". Note that the string contains the object identity and the port number that were used by the server. (Obviously, hard-coding object identities and port numbers into our applications is a bad idea, but it will do for now; we will see more architecturally sound ways of doing this in Chapter 35.)

3. The proxy returned by stringToProxy is of type Ice::ObjectPrx, which is at the root of the inheritance tree for interfaces and classes. But to actually talk to our printer, we need a proxy for a Printer interface, not an Object interface. To do this, we need to do a down-cast by calling PrinterPrxHelper.checkedCast. A checked cast sends a message to the server, effectively asking "is this a proxy for a Printer interface?" If so, the call returns a proxy of type Demo::Printer; otherwise, if the proxy denotes an interface of some other type, the call returns null.

4. We test that the down-cast succeeded and, if not, throw an error message that terminates the client.

5. We now have a live proxy in our address space and can call the printString method, passing it the time-honored "Hello World!" string. The server prints that string on its terminal.

Compiling the client looks much the same as for the server:

$ vbc /reference:Ice.dll /libpath:%ICE_HOME%\bin /reference:Printer.dll /out:client.exe Client.vb

Running Client and Server

To run client and server, we first start the server in a separate window:

$ server.exe

At this point, we won’t see anything because the server simply waits for a client to connect to it. We run the client in a different window:

$ client.exe
$

The client runs and exits without producing any output; however, in the server window, we see the "Hello World!" that is produced by the printer. To get rid of the server, we interrupt it on the command line for now. (We will see cleaner ways to terminate a server in Chapter 16.)

If anything goes wrong, the client will print an error message. For example, if we run the client without having first started the server, we get something like the following:

Ice.ConnectionRefusedException
    error = 0
   at IceInternal.ProxyFactory.checkRetryAfterException(LocalException ex, Reference ref, Int32 cnt) in c:\cygwin\home\m
ichi\src\ice\cs\src\Ice\ProxyFactory.cs:line 167
   at Ice.ObjectPrxHelperBase.handleException__(ObjectDel_ delegate, LocalException ex, Int32 cnt) in c:\cygwin\home\mic
hi\src\ice\cs\src\Ice\Proxy.cs:line 970
   at Ice.ObjectPrxHelperBase.ice_isA(String id__, Dictionary`2 context__, Boolean explicitContext__) in c:\cygwin\home\
michi\src\ice\cs\src\Ice\Proxy.cs:line 201
   at Ice.ObjectPrxHelperBase.ice_isA(String id__) in c:\cygwin\home\michi\src\ice\cs\src\Ice\Proxy.cs:line 170
   at Demo.PrinterPrxHelper.checkedCast(ObjectPrx b) in C:\cygwin\home\michi\src\ice\cs\demo\book\printer\generated\Prin
ter.cs:line 140
   at Client.Main(String[] args) in C:\cygwin\home\michi\src\ice\cs\demo\book\printer\Client.cs:line 23
Caused by: System.ComponentModel.Win32Exception: No connection could be made because the target machine actively refused
 it

Note that, to successfully run client and server, the VB run time must be able to locate the Ice.dll library. (Under Windows, one way to ensure this is to copy the library into the current directory. Please consult the documentation for your VB run time to see how it locates libraries.)

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1

This approach works not only with Visual Basic, but with any language that targets the .NET run time. However, ZeroC does not provide support for languages other than C# and Visual Basic.

2

Well, two lines, really: printing space limitations force us to break source lines more often than you would in your actual source files.