Handling exceptions inline

By default, the ActiveX proxy raises an ActiveX exception when an EAServer component method raises an exception or an internal error occurs. Visual Basic and most other ActiveX scripting tools do not allow you to handle these errors inline. Instead, control transfers to an error handler (specified by on error goto in Visual Basic) or to a system-wide error dialog box.

Inline exception handling can simplify the code that handles recoverable errors. For example, you can keep program logic that allows a user to retry a failed login in one place, rather than split into mainline code and the separate error handling code. Inline exception handling also allows you to handle errors explicitly in scripting tools that do not allow you to install user-coded error handlers.

The ActiveX proxy supports inline exception handling with Try, Catch, and End methods and an internal exception store. When an exception occurs with inline handling active, the proxy stores the error information rather than raising an ActiveX exception. Each component proxy object supports these methods and contains an exception store that is specific to that object. To handle exceptions inline, call the Try_, Catch_, and End_ methods as follows:

The Try_ and Catch_ methods do not have the same semantics of structured exception handling in Java or C++. In particular:

When you call the Catch_ method, you can check for exceptions of a specific type, or for exceptions of any type. To check for any exception, pass “...” as the exception type parameter.

The following example illustrates this style of exception handling:

barcomp.Try_
barcomp.methodThatRaisesException(1007)
Dim anyExcep As Object
If (barcomp.Catch_("...", anyExcep) = True) Then
    Dim excepType as String
    excepType = anyExcep.GetExceptionType
    if (StrComp(excepType, "Foo/NotValidIdException") == 0) then
       Dim invalidIdExcep as NotValidIdException
       set invalidIdExcep = anyExcep
       Dim id as integer
       Dim msg as String
       id = invalidIdExcep.id
       msg = invalidIdExcep.message
    Else if (StrComp(excepType, "Foo/NoAuthorizationException") == 0) then
        Dim noAuthorizationExcep as NoAuthorizationException
        set noAuthorizationExcep = anyExcep
        Dim user as String
        Dim cert as String
        user = noAuthorizationExcep.username
        cert = noAuthorizationExcep.certificate
    Else if (StrComp(excepType, "Jaguar/ClientException") == 0) then
        Dim systemExcep as SystemException
        set systemExcep = excep
        Dim code as integer
        Dim msg as String
        code = systemExcep.code
        msg = systemExcep.message
    End if
    Else
        ' No Exception has occurred. Proceed
End If

Exception datatypes are used with the Try_ method when handling exceptions inline. The ActiveX proxy includes predefined system exceptions that correspond to the standard CORBA system exceptions. User-defined exceptions that are declared in an IDL module are also mapped to ActiveX types.

interface SystemException
{
  long code;      // numeric error code
  string message; // text error message
};

Unlike user-defined exceptions, a component method can throw system exceptions that are not listed in the raises clause of the IDL method signature. The C++ and ActiveX client runtime engines may also raise system exceptions when errors occur in the processing of a method invocation.

In the ActiveX proxy, system exceptions are mapped to the interface SystemException with the following properties and methods:

exception InvalidValueException
{
  string message;
  string value;
};

User-defined exceptions can be defined within an IDL module or interface. The IDL method signature for a component method must list user-defined exceptions thrown by the method in the raises clause. A method cannot throw user-defined exceptions that are not listed in the raises clause.

In ActiveX, the IDL exception maps to an interface with the following properties and methods:

module/exception 

Where module is the IDL module name and exception is the IDL exception type. For example, “CtsSecurity/No Certificate Exception”. The exception identifier for an exception defined in an interface is:

module/interface/exception 

Where interface is the IDL interface name.

If your program uses a proxy object in multiple threads and handles exceptions inline, you must call the Duplicate_ method to obtain a copy of the proxy object for use in each thread. Duplicate_ has the following syntax:

Object Duplicate_

Duplicate_ returns a proxy instance of the same type as the original.