How to fix the assembly binding redirect problem in Azure Functions

In one of our recent gigs, we had to use MS Azure Functions. I’ll discuss about the architecture and how we’re using Azure Functions in our SaaS cloud solution in another post. In this post though, I’m going to talk about the problem we had in Azure Functions and propose a solution for that.

Assembly Binding Redirect issue:

As you may know, in .NET, there is a feature called “Assembly Binding Redirect” that works for any type of application. This binding redirect thing, is used when your app depends on multiple assemblies (dlls) that are themselves using different versions of the same assembly.  Case in point, let’s say you’re using Microsoft.Rest NuGet package that depends on Newtonsoft.Json v9.0.0.0 and also using Box.V2 NuGet package that depends on Newtonsoft.Json v10.0.0.0 . Therefore, at the run time, we need something to redirect all Newtonsoft.json dependencies to redirect to a single version (say v10.0.0.0). The reason being we can’t have multiple versions of the same assembly at the run time. For more info about the binding redirect have a look into the following link :
https://docs.microsoft.com/en-us/dotnet/framework/configure-apps/redirect-assembly-versions

You can simply do assembly binding redirect in all types of .NET apps. You just need to add a section to your app.config or web.config file (in case of web apps) just like the following:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <dependentAssembly>
        <assemblyIdentity name="Newtonsoft.Json" publicKeyToken="30ad4fe6b2a6aeed" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-10.0.0.0" newVersion="10.0.0.0" />
      </dependentAssembly>
    </assemblyBinding>
  </runtime>
</configuration>

As you see, the setting redirects all references with whatever versions (from 0.0.0.0 up to 10.0.0.0) of the Newtonsoft.Json assembly to the version 10.0.0.0 , simple as that!

In the Azure Functions though, we can’t do that; at least at the time of writing this post. So if you add an app.config to your Azure Function Project, it wouldn’t take effect, as if there is no app.config. The reason being, the way that Functions run on Azure is a bit different from normal .NET apps, as they themselves run over webjobs platform. Check out the following ticket in GitHub to see how come binding redirect doesn’t work:
https://github.com/Azure/azure-webjobs-sdk-script/issues/992

It you’re working on a real project, this problem would be quite annoying, as the code doesn’t actually work once your project’s dependencies are pointing to multiple versions of the same assembly. You simply get stuck, just as we did!

How to resolve it:

However, at the time of writing this post, the problem is not resolved by Microsoft, there is a workaround for that. Let’s say we want to add something to our application settings, which is the “local.settings.json” in Azure Functions and could be accessed through the Azure Portal, just as shown below:

Function Application Settings

So we just added a new field named “BindingRedirects” to the settings and set its value like this:

[ { “ShortName”: “Newtonsoft.Json”, “RedirectToVersion”: “10.0.0.0”, “PublicKeyToken”: “30ad4fe6b2a6aeed” } ]

Which means redirect all the references to the “Newtonsoft.Json” to version 10.0.0.0

And here’s what our local.settings.json looks like:

{
  "IsEncrypted": false,
  "Values": {
    "AzureWebJobsStorage": "UseDevelopmentStorage=true",
    "BindingRedirects": "[ { \"ShortName\": \"Newtonsoft.Json\", \"RedirectToVersion\": \"10.0.0.0\", \"PublicKeyToken\": \"30ad4fe6b2a6aeed\" } ]"
  }
}

using System;
using System.Collections.Generic;
using System.Globalization;
using System.IO;
using System.Reflection;
using System.Runtime.Serialization.Json;
using System.Text;
namespace Connectors.AzureFunctions.Application
{
    public static class AssemblyBindingRedirectHelper
    {

        ///<summary>
        /// Reads the "BindingRedirecs" field from the app settings and applies the redirection on the
        /// specified assemblies
        /// </summary>

        public static void ConfigureBindingRedirects()
        {
            var redirects = GetBindingRedirects();
            redirects.ForEach(RedirectAssembly);
        }

        private static List<BindingRedirect> GetBindingRedirects()
        {
            var result = new List<BindingRedirect>();
            var bindingRedirectListJson = Environment.GetEnvironmentVariable("BindingRedirects");
            using (var memoryStream = new MemoryStream(Encoding.Unicode.GetBytes(bindingRedirectListJson)))
            {
                var serializer = new DataContractJsonSerializer(typeof(List<BindingRedirect>));
                result = (List<BindingRedirect>)serializer.ReadObject(memoryStream);
            }
            return result;
        }

        private static void RedirectAssembly(BindingRedirect bindingRedirect)
        {
            ResolveEventHandler handler = null;
            handler = (sender, args) =>
            {
                var requestedAssembly = new AssemblyName(args.Name);
                if (requestedAssembly.Name != bindingRedirect.ShortName)
                {
                    return null;
                }
                var targetPublicKeyToken = new AssemblyName("x, PublicKeyToken=" + bindingRedirect.PublicKeyToken).GetPublicKeyToken();
                requestedAssembly.SetPublicKeyToken(targetPublicKeyToken);
                requestedAssembly.Version = new Version(bindingRedirect.RedirectToVersion);
                requestedAssembly.CultureInfo = CultureInfo.InvariantCulture;
                AppDomain.CurrentDomain.AssemblyResolve -= handler;
                return Assembly.Load(requestedAssembly);
            };
            AppDomain.CurrentDomain.AssemblyResolve += handler;
        }

        public class BindingRedirect
        {
            public string ShortName { get; set; }
            public string PublicKeyToken { get; set; }
            public string RedirectToVersion { get; set; }
        }
    }
}

The code simply reads a setting field named “bindingRedirects” from the Function app settings, deserialize it to  List of BindingRedirects and goes through the list and hooks a new method to the “AppDomain.CurrentDomain.AssemblyResolve” event. That event gets triggered once a new assembly gets resolved. Whenever you write a code that uses an external assembly, as soon it run the code that calls a type from the assembly, it triggers that event and gets resolved and loaded. If the assembly is in our binding redirect list, then we  load the version specified in the settings, otherwise we do nothing.

We just need to call this method only once, and before running any other code in the function app. So what I did was creating this class as a helper, it just guaranties that the code above runs only once.

public static class ApplicationHelper
    {
        private static bool IsStarted = false;
        private static object _syncLock = new object();
        ///<summary>
        /// Sets up the app before running any other code
        /// </summary>

        public static void Startup()
        {
            if (!IsStarted)
            {
                lock (_syncLock)
                {
                    if (!IsStarted)
                    {
                        AssemblyBindingRedirectHelper.ConfigureBindingRedirects();
                        IsStarted = true;
                    }
                }
            }
        }
    }

Then in your Azure Function, simple call this helper. But we need to make sure that it runs before any other code, so I put it in the static constructor of the class. I’m assuming that you’re using the project template of Visual Studio 2017 Azure Function tools for development.

using System.Linq;
using System.Net;
using System.Net.Http;
using Microsoft.Azure.WebJobs;
using Microsoft.Azure.WebJobs.Extensions.Http;
using Microsoft.Azure.WebJobs.Host;
using System.Threading.Tasks;
using Newtonsoft.Json;
using System.Collections.Generic;

namespace Connectors.AzureFunctions
{
    public static class EventProcessor
    {
        static EventProcessor()
        {
            ApplicationHelper.Startup();
        }

         [FunctionName("EventProcessor")]
        public static async Task<HttpResponseMessage> Run(
            [HttpTrigger(AuthorizationLevel.Function, "get", "post", Route = null)]HttpRequestMessage req,
            TraceWriter log)
        {
            // Method's body, in our case, let's say it's a code which is using Newtonsoft.Json
        }
    }

}

Now we should be good, and if you need to add more binding redirects, then no need to touch the code, just add the new redirect to the settings, sit back and enjoy.

Hope this workaround resolves your binding redirect issues in the Function apps and saves some time and effort!

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