6.5 Retrying Code

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Sometimes it is necessary to rerun code that has triggered an error. Unfortunately, there is no good way to rerun code by using structured exception handling. Example 6-9 shows one method that uses nested Try blocks, which is ideal if you need to retry code only a few times.

Example 6-9. Retrying a method

 Public Shared Sub Main( )       Try     RetryMethod( )   Catch e As Exception         Try       RetryMethod( )     Catch e As Exception       'Fail     End Try       End Try     End Sub 

Nested Try blocks are great unless you want to retry your code more than a few times. Beyond that, reading the code becomes more difficult. If someone has to scroll the code window out to column 420 to see what you have donewell, they will laugh at you and tell people about it on the elevator.

Try blocks are re-entrant , so you can use a Goto to jump back into a Try block that already executed. Example 6-10 demonstrates this technique.

Example 6-10. Retrying code after a failure

 Imports System     Public Class App       Public Shared Sub Main( )         Dim attempts As Integer = 0     Dim maxRetries As Integer = 5         Try       'Code to be retried n times retry:       attempts += 1       Console.WriteLine("Attempt {0}", attempts.ToString( ))           'Simulate error - jumps to Catch block       Throw New Exception( )         Catch e As Exception           If attempts = maxRetries Then         GoTo done       End If           GoTo retry done:     End Try       End Sub     End Class 

This code does not flow well, and it is difficult to read. It exists just in case someone out there decides to be clever. Stop trying to be clever when you code. There is a better way to retry a path of execution; you can use recursion from a Catch block and monitor how many attempts were made on the method with a private instance variable, which is shown in Example 6-11.

The Goto Is Obsolete

Don't believe the hype. Before structured error handling, the Goto did have a place in structured code. When several resources, such as database connections and file handles, were allocated, the Goto allowed a single "cleanup" location. Consider the following pseudocode:

 Public Sub PseudoCode( )     Dim conn As New OleDbConnection(...)     Dim fStream As FileStream = File.Open(...)         If error Then         Goto cleanup     End If         If error Then         Goto cleanup     End If         If error Then         Goto cleanup     End If     cleanup:     conn.Close( )     fstream.Close( ) End Sub 

This pattern allows cleanup to be performed in a single place rather than being repeated whenever an error occurs. However, this pattern is no longer necessary because with structured exception handling, cleanup code can be placed in a Finally block.

Example 6-11. A better way to retry code

 Imports System     Public Class Retry       Private attempts As Integer = 0   Private Const maxRetries As Byte = 5       Public Sub Go( )         Try       'Code to retry0       Console.WriteLine("Attempt #{0}", _           attempts.ToString( ))       'Simulate error       Throw New Exception( )     Catch e As Exception When attempts < maxRetries       attempts += 1       Go( ) 'Call method again     Catch e As Exception       'Failed so handle here     End Try         'Reset attempt counter     attempts = 0       End Sub     End Class     Public Class App   Public Shared Sub Main( )     Dim r As New Retry( )     r.Go( )     Console.ReadLine( )   End Sub End Class 

Retrying code multiple times is considerably easier with the conditional handling of exceptions. Remember, exceptions are caught in the order they are received. In Example 6-11, the first exception handler's When clause provides a simple alternative to rerunning a block of code:

 Try     Go( ) 'Code that you want to re-run on failure Catch e As Exception When attempts < maxRetries     attempts += 1     Go( ) 'Call method again Catch e As Exception     'Failed so handle here End Try 

After the internal counter exceeds its limit, the condition on the first Catch block is no longer valid, and the second handler catches any exception. The example ignores the exception, but in this situation, you could probably log the error as discussed earlier in the chapter.

Use unstructured exception handling only when migrating from earlier versions of VB to the .NET environment. New projects should always use structured exception handling.

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