There's more...

Ensure that you have all the following namespaces in the class:

using System.Net;
using System.Net.Http;
using System.Threading.Tasks;
using Microsoft.Azure.WebJobs;
using Microsoft.Azure.WebJobs.Extensions.Http;
using Microsoft.Azure.WebJobs.Host;
using System.Data.SqlClient;
using System.Data;
using System;
using Microsoft.Azure.Services.AppAuthentication;