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C# 9 has some amazing features. Azure Functions are have two modes: isolated and in-process. Whilst isolated supports .NET 5 (and hence C# 9), in-process supports .NET Core 3.1 (C# 8). This post shows how we can use C# 9 with in-process Azure Functions running on .NET Core 3.1.

title image showing name of post and the Azure Functions logo

Azure Functions: in-process and isolated#

Historically .NET Azure Functions have been in-process. This changed with .NET 5 where a new model was introduced named "isolated". To quote from the roadmap:

Running in an isolated process decouples .NET functions from the Azure Functions host—allowing us to more easily support new .NET versions and address pain points associated with sharing a single process.

However, the initial launch of isolated functions does not have the full level of functionality enjoyed by in-process functions. This will happen, according the roadmap:

Long term, our vision is to have full feature parity out of process, bringing many of the features that are currently exclusive to the in-process model to the isolated model. We plan to begin delivering improvements to the isolated model after the .NET 6 general availability release.

In the future, in-process functions will be retired in favour of isolated functions. However, it will be .NET 7 (scheduled to ship in November 2022) before that takes place:

the Azure Functions roadmap image illustrating the future of .NET functions taken from

As the image taken from the roadmap shows, when .NET 5 shipped, it did not support in-process Azure Functions. When .NET 6 ships in November, it should.

In the meantime, we would like to use C# 9.

Setting up a C# 8 project#

We're have the Azure Functions Core Tools installed, so let's create a new function project:

func new --worker-runtime dotnet --template "Http Trigger" --name "HelloRecord"

The above command scaffolds out a .NET Core 3.1 Azure function project which contains a single Azure function. The --worker-runtime dotnet parameter is what causes an in-process .NET Core 3.1 function being created. You should have a .csproj file that looks like this:

<Project Sdk="Microsoft.NET.Sdk">  <PropertyGroup>    <TargetFramework>netcoreapp3.1</TargetFramework>    <AzureFunctionsVersion>v3</AzureFunctionsVersion>  </PropertyGroup>  <ItemGroup>    <PackageReference Include="Microsoft.NET.Sdk.Functions" Version="3.0.11" />  </ItemGroup>  <ItemGroup>    <None Update="host.json">      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>    </None>    <None Update="local.settings.json">      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>      <CopyToPublishDirectory>Never</CopyToPublishDirectory>    </None>  </ItemGroup></Project>

We're running with C# 8 and .NET Core 3.1 at this point. What does it take to get us to C# 9?

What does it take to get to C# 9?#

There's a great post on Reddit addressing using C# 9 with .NET Core 3.1 which says:

You can use <LangVersion>9.0</LangVersion>, and VS even includes support for suggesting a language upgrade.

However, there are three categories of features in C#:

  1. features that are entirely part of the compiler. Those will work.

  2. features that require BCL additions. Since you're on the older BCL, those will need to be backported. For example, to use init; and record, you can use

  3. features that require runtime additions. Those cannot be added at all. For example, default interface members in C# 8, and covariant return types in C# 9.

Of the above, 1 and 2 add a tremendous amount of value. The features of 3 are great, but more niche. Speaking personally, I care a great deal about Record types. So let's apply this.

Adding C# 9 to the in-process function#

To get C# into the mix, we want to make two changes:

  • add a <LangVersion>9.0</LangVersion> to the <PropertyGroup> element of our .csproj file
  • add a package reference to the IsExternalInit

The applied changes look like this:

<Project Sdk="Microsoft.NET.Sdk">  <PropertyGroup>    <TargetFramework>netcoreapp3.1</TargetFramework>+    <LangVersion>9.0</LangVersion>    <AzureFunctionsVersion>v3</AzureFunctionsVersion>  </PropertyGroup>  <ItemGroup>    <PackageReference Include="Microsoft.NET.Sdk.Functions" Version="3.0.11" />+    <PackageReference Include="IsExternalInit" Version="1.0.1" PrivateAssets="all" />  </ItemGroup>  <ItemGroup>    <None Update="host.json">      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>    </None>    <None Update="local.settings.json">      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>      <CopyToPublishDirectory>Never</CopyToPublishDirectory>    </None>  </ItemGroup></Project>

If we used dotnet add package IsExternalInit, we might be using a different syntax in the .csproj. Be not afeard - that won't affect usage.

Making a C# 9 program#

Now we can theoretically use C# 9.... Let's use C# 9. We'll tweak our HelloRecord.cs file, add in a simple record named MessageRecord and tweak the Run method to use it:

using System;using System.IO;using System.Threading.Tasks;using Microsoft.AspNetCore.Mvc;using Microsoft.Azure.WebJobs;using Microsoft.Azure.WebJobs.Extensions.Http;using Microsoft.AspNetCore.Http;using Microsoft.Extensions.Logging;using Newtonsoft.Json;
namespace tmp{    public record MessageRecord(string message);
    public static class HelloRecord    {        [FunctionName("HelloRecord")]        public static async Task<IActionResult> Run(            [HttpTrigger(AuthorizationLevel.Function, "get", "post", Route = null)] HttpRequest req,            ILogger log)        {            log.LogInformation("C# HTTP trigger function processed a request.");
            string name = req.Query["name"];
            string requestBody = await new StreamReader(req.Body).ReadToEndAsync();            dynamic data = JsonConvert.DeserializeObject(requestBody);            name = name ?? data?.name;
            var responseMessage = new MessageRecord(string.IsNullOrEmpty(name)                ? "This HTTP triggered function executed successfully. Pass a name in the query string or in the request body for a personalized response."                : $"Hello, {name}. This HTTP triggered function executed successfully.");
            return new OkObjectResult(responseMessage);        }    }}

If we kick off our function with func start:

screenshot of the output of the HelloRecord function

We can see we can compile, and output is as we might expect and hope. Likewise if we try and debug in VS Code, we can:

screenshot of the output of the HelloRecord function

Best before...#

So, we've now a way to use C# 9 (or most of it) with in-process .NET Core 3.1 apps. This should serve until .NET 6 ships in November 2021 and we're able to use C# 9 by default.