title: Error binding Blob to IAsyncCollector when binding to output blob in Async method
body: <p>I'm trying to bind to a blob output in an Async method following this post: <strong><a href="https://stackoverflow.com/questions/40409344/how-can-i-bind-output-values-to-my-async-azure-function">How can I bind output values to my async Azure Function?</a></strong></p>
  
  <p>I have multiple output bindings so just returning is not an option </p>
  
  <pre><code>public static async Task&lt;HttpResponseMessage&gt; Run(HttpRequestMessage req, IAsyncCollector&lt;string&gt; collection, TraceWriter log)
  {
      if (req.Method == HttpMethod.Post) 
      {
          string jsonContent = await req.Content.ReadAsStringAsync();
  
          // Save to blob 
          await collection.AddAsync(jsonContent);
  
          return req.CreateResponse(HttpStatusCode.OK);
      }
      else 
      {
          return req.CreateResponse(HttpStatusCode.BadRequest);
  
      }
  }
  </code></pre>
  
  <p>My binding for the blob is :</p>
  
  <pre><code>{
    "bindings": [
      {
        "authLevel": "function",
        "name": "req",
        "type": "httpTrigger",
        "direction": "in"
      },
      {
        "name": "$return",
        "type": "http",
        "direction": "out"
      },
      {
        "type": "blob",
        "name": "collection",
        "path": "testdata/{rand-guid}.txt",
        "connection": "test_STORAGE",
        "direction": "out"
      }
    ],
    "disabled": false
  }
  </code></pre>
  
  <p>But whenever I do this I get the following:</p>
  
  <blockquote>
    <p>Error: Function ($WebHook) Error:
    Microsoft.Azure.WebJobs.Host: Error indexing method
    'Functions.WebHook'. Microsoft.Azure.WebJobs.Host: Can't bind
    Blob to type
    'Microsoft.Azure.WebJobs.IAsyncCollector`1[System.String]'</p>
  </blockquote>