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Let's build an error handling aspect: part 5

April 11, 2014 mgroves 0 Comments
Tags: LetsBuildAnAspect postsharp exceptionless

In the last post, I added some compile time validation to make sure that the aspect is being used in the correct places. Today, we're going to look at actually logging the exception.

Remember early on when I said that this aspect is not generally applicable? For this part, it's especially true, since I'll be using a specific logging library + service: Exceptionless.

The application that I'm working on is meant to be deployed to an Azure web site. Because of this, I have to think a little differently about logging. You can still log to the file system, of course, but on a cloud service that may not be the best idea. You could log to table storage on Azure Storage. You could also log to a normal database. However, I wanted to use something that: a) requires very little work, b) provides some useful monitoring tools that I don't have to build, c) is very low cost or free. So, based on my research and an endorsement from Mark Greenway, I decided to explore Exceptionless.

Using Exceptionless is as simple as installing a NuGet package, pasting in your API key, and then using an extension method.

Here's how I could use Exceptionless in my aspect:

This is very similar to what I did initially, but since this project is in development, I made a few adjustments along the way to make my life easier.

SQL exceptions. Especially during development, it's very likely that I'll screw up a SQL query, forget to run a migration script, etc. Additionally, since I'm using constraints in my database, I want to handle the case where a user is trying to delete an entity and a constraint is preventing them. So I modified the aspect some more:

The way I'm checking for "constraint" exceptions feels very hacky to me, but I haven't found a better way yet.

NotImplementedException. Again, since I'm still in active development, often times a NotImplementedException will pop up. For these exceptions, I want them to crash right away, and I also don't need to worry about logging.

Now that SQL exceptions and not-implemented exceptions are handled on their own, any other exception is one that I still want to log and output to the UI in a friendly, non-yellow screen way. During development, I definitely want to see the full exception message, but that's something that will definitely need to be revisited before going into production. Back to the list:

  • This only works for ITerritoryService
  • Swallowing exceptions: we should at least be logging the exception, not just ignoring it. <- We are now logging exceptions!
  • If the method has a return type, then it's returning null. If it's meant to return a reference type, that's probably fine, but what if it's returning a value type?
  • What if it's returning a collection, and the UI assumes that it will get an empty collection instead of a null reference?
  • Do we really want to return the exception message to the user? <- We'll revisit this later.
  • What if I forget to use ServiceBase as a base class on a new service?



Matthew D. Groves

About the Author

Matthew D. Groves lives in Central Ohio. He works remotely, loves to code, and is a Microsoft MVP.

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