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CHAPTER 14 EXCEPTIONS
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The identifier for the exception in a catch clause is optional, but if you omit the identifier, then you are unable to access the members of the exception that is being handled. Listing 14-6 demonstrates a try statement with two catch clauses, one of which omits the exception identifier. Listing 14-6. Omitting the Exception Identifier in a Specific catch Clause try { // statements likely to cause exceptions } catch (NullReferenceException ex) { Console.WriteLine("Message: {0}", ex.Message); } catch (ArgumentOutOfRangeException) { // code which handles an ArgumentOutOfRangeException } The first catch clause defines an identifier, ex, for the exception. This allows the handler statements to refer to the exception using this identifier for example, reading the Message property from the exception by calling ex.Message. The second catch clause doesn t define an identifier and so cannot make such a reference.
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Using General catch Clauses
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A try statement can have at most one general catch clause, and it must be the last catch clause in the list. General catch clauses can be used as an effective backstop to prevent exceptions from propagating up the call stack to the default handler. Despite this, general clauses are rarely used, because they don t allow you to access the exception that is being thrown, which is a severe limitation. A more common solution is to use a specific catch clause for the System.Exception type; see the previous section for details.
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Omitting catch Clauses
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All the examples in this chapter so far have included at least one catch clause, but in fact, you can define try statements that don t have any catch clauses at all. Listing 14-7 demonstrates such a try statement, which has no catch clauses but does have a finally clause, which I describe later in the chapter. Listing 14-7. Omitting catch Clauses from a try Statement try { // statements likely to cause exceptions
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CHAPTER 14 EXCEPTIONS
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} finally { // finally statements go here... } Any exceptions caused by the code statements enclosed by the try statement will go unhandled, and the runtime will have to search further up the call stack, as discussed earlier in the chapter. This is a very uncommon use of the try statement, which usually includes at least one catch clause.
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Nesting try Statements
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You can define one try statement so that it is enclosed by another. In the event of an exception, the runtime will look first at the inner try statement and then work its way out, trying to find a match for a catch clause. Listing 14-8 contains an example. Listing 14-8. Nesting try Statements try { try { // statements likely to cause exceptions } catch (Exception) { // handler statements for the inner try statement } // more code statements } catch (Exception) { // handler statements for the outer try statement } The effect of this example is to define a set of catch clauses for one group of code statements and a different set for others. In Listing 14-8, both try statements have catch clauses for the same exception type, but this need not be the case, and if an exception is unhandled by the inner catch clause, the runtime will examine the outer try statement and use any matching catch clauses it can find.
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Using Exception Members
When you include an exception identifier in a catch clause, you can refer to the members of the exception using that identifier, just as you would any local variable. Table 14-2 describes the most useful properties of the System.Exception class. Some of these members are demonstrated here and others in the following sections.
CHAPTER 14 EXCEPTIONS
Table 14-2. Useful Members of the System.Exception Class
Member
Data
Description
Returns an IDictionary that can be used to get and set name/value pairs. Returns the exception that caused this exception, or null if there is no exception Returns a message that describes the current exception Gets or sets the name of the object that caused the exception Gets a string representation of the call stack Gets the details of the method that caused the exception Returns the root exception in a chain of exceptions Returns details of the type message and stack trace of the exception
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