Tuesday, 21 January 2014

Xero API .NET Client - Implementing your own ITokenRepository

We recently integrated our FoodStorm product with Xero, and we decided to use their .NET API. Most of it is pretty straightforward - except for dealing with access tokens in .NET as their documentation is lacking somewhat.

So here is a bit of a brain-dump of my experiences...

To perform any API operation, you need to first instantiate a Session. In our case we have Partner access, so we need to create a XeroApiPartnerSession. The constructor requires an ITokenRepository (it also requires Certificates - but that's for another blog post!).

So since you can't create an instance of an Interface, you need to create your own class that implements ITokenRepository, and inside that class it's up to you to deal with the loading & saving of access tokens.

When I first saw this, I thought it was overkill - why couldn't I just pass in my access token?!? But I delved more into the API I learned that access tokens expire after 30 minutes - even with Partner access. This means you need to Renew your access token periodically. The good news is that if you implement an ITokenRepository, all this complexity is fully handed by the .NET API, so it's a pretty good design after all.

So onto creating a Token Repository... The ITokenRepository interface looks like this:

As you can probably guess, you need to make the Get methods return your tokens, and your Set methods need to save the tokens. My initial idea was just to Serialize the AccessToken and RequestToken classes to a file - easy peasy - until I realised those classes weren't marked as Serializable...

So that meant I would need to store the values of the properties in the Set methods, and then re-create the classes in the Get methods. And that's what I did:

For simplicity sake, I decided to serialise all the required properties to my own JSON object, as I didn't see the need for specific DB columns etc - but that's up to you. However note that ALL of the properties I am save & loading here are required! If you miss any (which I initially did), things will not work correctly within the API.