title: Azure Functions: host error has occurred during startup operation Could not load file
body: <p>I am trying to use Microsoft.AspNetCore.Authentication.Facebook with an Azure functions project. I created a completely clean .net core 3.1 Azure Function project only with the following dependencies:</p>
  
  <pre><code>Microsoft.NET.Sdk.Functions 3.0.7
  Microsoft.Azure.Functions.Extensions 1.0.0
  Microsoft.AspNetCore.Authentication.Facebook 3.1.5
  </code></pre>
  
  <p>In the startup file I have the following code:</p>
  
  <pre><code> public override void Configure(IFunctionsHostBuilder builder)
   {           
       facebookOptions.AppId = Environment.GetEnvironmentVariable("Authentication:Facebook:AppId");
       facebookOptions.AppSecret = Environment.GetEnvironmentVariable("Authentication:Facebook:AppSecret");
   });
  </code></pre>
  
  <p>When I run the application I get the following error in the console window:</p>
  
  <pre><code>&gt; A host error has occurred during startup operation Could not load file
  &gt; or assembly 'Microsoft.AspNetCore.Authentication.Facebook,
  &gt; Version=3.1.5.0, Culture=neutral, PublicKeyToken='. The system cannot
  &gt; find the file specified.
  </code></pre>
  
  <p>Any idea what could be wrong?</p>