title: ProviderException when using a custom output cache provider RedisOutputCacheProvider
body: <p>I have installed the nuget package <code>Microsoft.Web.RedisOutputCacheProvider</code> for use with Azure Web App running on multiple server instances following the instructions here <a href="https://msdn.microsoft.com/en-us/library/azure/dn798898.aspx" rel="nofollow">https://msdn.microsoft.com/en-us/library/azure/dn798898.aspx</a> and are receiving the following error:</p>
  
  <pre><code>When using a custom output cache provider like 'MyRedisOutputCache', only the following expiration policies and cache features are supported:  file dependencies, absolute expirations, static validation callbacks and static substitution callbacks.
  </code></pre>
  
  <p>The configuration setting is as follows:</p>
  
  <pre><code>&lt;system.web&gt;
    &lt;caching&gt;
      &lt;outputCache defaultProvider="MyRedisOutputCache"&gt;
        &lt;providers&gt;
          &lt;add name="MyRedisOutputCache" 
               type="Microsoft.Web.Redis.RedisOutputCacheProvider" 
               port="1234" 
               host="[my host]" 
               accessKey="[my access key]" 
               ssl="true" /&gt;
        &lt;/providers&gt;
      &lt;/outputCache&gt;
    &lt;/caching&gt;
  &lt;/system.web&gt;
  </code></pre>
  
  <p>I've tried setting the <code>connectionString</code> to a valid StackExchange.Redis connection string instead of setting the individual port/host etc... but still receive the same error.</p>
  
  <p>We also have the <code>Microsoft.Web.RedisSessionStateProvider</code> running in the same web app using the same settings with no issues. </p>
  
  <p>Any help would be appreciated. </p>