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Add Custom Data to Exceptions

When it comes to debugging problems with your code, a good Exception object is your most valuable tool. I've talked before about how why returning information about an exception using the InnerException object will let you find out what the real problem is without giving away secrets about your application.

I've also discussed how to ensure that your server-side errors aren't masked by the generic 500 "something's gone wrong" HTTP error. I've even gone as far as to suggest that it might be useful to create your own custom exception object.

Some people might conclude that my programs must throw a lot of errors but that's not true. My concern is simple: Knowing precisely what went wrong when your program blows up helps you figure out what the problem is. Furthermore, knowing what data exposed the problem is often critical to solving the problem. To help you out with that, the Exception object provides a dictionary where you can tuck away data about an error that programs processing the error can use -- it's the Exception object's Data property.

The following code adds an item to an Exception object's Data collection with a key of CustomerId and sets that key to a value. The code then throws an Exception of its own, tucking the original exception (with its additional information) into the InnerException property:

Try
   ...code that might throw an exception...
Catch ex As Exception
   ex.Data.Add("CustomerId", custId)
   Throw New Exception("Failure in processing Customer", ex)
End Try

Alternatively, you can create your own Exception object and add to its Data collection:

Try
   ...code that might throw an exception...
Catch ex As Exception
   Dim myEx As New Exception("Failure in processing Customer", ex)
   myEx.Data.Add("PersonId", pers.FirstName)
   Throw myEx
End Try

The code that processes an Exception doesn't have to know precisely what's in the Data collection in order to report on it. Instead, you can just write out everything in the Data collection using code like this:

For Each key In ex.Data.Keys
  Debug.Print(key & ": " & ex.Data(key))
Next

There are two things to avoid here. First, make sure that retrieving the information you're going to put in the Data collection won't itself trigger an exception (don't try to set a key to the property of a potentially null reference, for example). Second, don't add to the collection of any Exception object that you haven't just created because you might overwrite a Data value that was set elsewhere.

Posted by Peter Vogel on 02/23/2016 at 12:11 PM


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