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In this example, the statement
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1/$null
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was executed. $null is treated like zero in integer expressions, causing a division-byzero exception to occur. When this happens, control transfers to the statement list in the body of the trap statement. In this case, it just writes Got it! which we see in the output. We also see that the error message is still displayed, even though we
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trap keyword Type of exception to catch (may be omitted)
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trap { }
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[<exceptionType >] <statementList > Body of the trap statement
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Figure 9.1 The syntax of the trap statement
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DEALING WITH ERRORS THAT TERMINATE EXECUTION
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trapped this exception. This is a significant point. What happens after a trap handler is complete depends on how the statement terminates. If the body of the statement simply exits normally then an error object will be written to the error stream, and, depending on the setting of $ErrorActionPreference, either the exception will be rethrown or execution will continue at the statement after the statement that caused the exception. This is what we saw in the previous example. To make this point clearer, let s add another statement after the one that caused the error:
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PS (2) > trap { "Got it!" } 1/$zero; "LAST" Got it! Attempted to divide by zero. At line:1 char:22 + trap { "Got it!" } 1/$ <<<< zero; "LAST" LAST
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We see the error message; but following it, we see output from the last statement. The interpreter s behavior after you leave the trap handler can be controlled by the break and continue keywords. (See chapter 6 for other uses of these keywords.) Let s look at break first. Here s the example again, but this time we ll terminate the trap block with break.
PS (3) > trap { "Got it!"; break } 1/$zero; "LAST" Got it! Attempted to divide by zero. At line:1 char:30 + trap { "Got it!"; break } 1/$ <<<< zero; "LAST"
We see the error record, but we don t see the output LAST because after the trap block exited, the error was rethrown as a terminating error instead of resuming execution. The other modification to the trap flow control is to use the continue statement.
PS (4) > trap { "Got it!"; continue Got it! LAST } 1/$zero; "LAST"
This time, we see the see the output from the trap block and from the LAST statement, but no error record. Exiting a trap block is approximately equivalent to the error action preference silently continue . There is one other feature available in the trap block itself. The exception that was trapped is available in the trap block in the $_ variable. Here s the example, but with the output of the trap statement showing the value in $_ as a string.
PS (5) > trap { "Got it: $_"; continue Got it: Attempted to divide by zero. } 1/$zero;
In this case, the output is the ToString() of the exception. However, $_ is not an exception; it s an error record, so the trap handler has full access to all of the information in the error handler. Let s verify the type of this object.
ERRORS, EXCEPTIONS, AND SCRIPT DEBUGGING
PS (6) > trap { "Got it: " + $_.gettype(); continue Got it: System.Management.Automation.ErrorRecord
} 1/$zero;
In the trap block in this example, we re displaying the type of the value in $_. Let s look at a somewhat more complex example. We said earlier that control transfers to the next statement after the one that caused the exception. That s not quite true. It transfers to the next statement in the same scope as the trap statement. In this example, we ll use scriptblocks to create the two scopes.
PS (7) > &{ >> trap {"TRAP"}
Here s the trap block in the outer scope.
>> &{
Now create an inner scope that will emit a number of strings.
>> "one" >> "two" >> 1/$null
Part of the way through, throw an exception.
>> >> >> >> "three" "four" } "OUTERBLOCK"
Back in the output block, write out the string "OUTERBLOCK" so we ll see what s happening.
>> } >> one two TRAP Attempted to divide by zero. At line:6 char:3 + 1/$ <<<< null OUTERBLOCK
Look at the output that was produced. You can see the first couple of numbers printed and then the exception, but look where execution resumed at the first statement outside the block. This pattern allows you to skip entire sections of code instead of a single line. It essentially mimics the try/catch pattern found in other languages such as C#. Having mastered catching exceptions, let s look at how to throw our own.
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