Deserialize message in AuthorizationFilterAttribute
Say I have a xml like the following which gets posted to an api controller:

    <request>
    <header><api_key>Somekey</api_key></header>
    </request>

If I want to validate the api key in a custom AuthorizationFilterAttribute like so: 

    public override void OnAuthorization(HttpActionContext actionContext)
    {
        // Deserialize the posted message and validate the api key.
    }

How can I do that without having to manually deserialize it?
Deserialize message in AuthorizationFilterAttribute
===

Say I have a xml like the following which gets posted to an api controller:

    <request>
    <header><api_key>Somekey</api_key></header>
    </request>

If I want to validate the api key in a custom AuthorizationFilterAttribute like so: 

    public override void OnAuthorization(HttpActionContext actionContext)
    {
        // Deserialize the posted message and validate the api key.
    }

How can I do that without having to manually deserialize it?