Concept example: Hosting an AspNetCore WebAPI inside an Azure Functions

HttpRequests.cs

using System.Configuration;
using System.IO;
using System.Net.Http;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Azure.WebJobs;
using Microsoft.Azure.WebJobs.Extensions.Http;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.TestHost;
using Microsoft.Azure.WebJobs.Host;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Newtonsoft.Json;

namespace FunctionApp1
{
    public static class HttpRequests
    {
        private static TestServer InternalTestServer;

        static HttpRequests()
        {
            var functionPath = new FileInfo(typeof(HttpRequests).Assembly.Location).Directory.FullName;
            Directory.SetCurrentDirectory(functionPath);
            InternalTestServer = CreateServer();
        }

        private static TestServer CreateServer()
        {
            var functionPath = new FileInfo(typeof(HttpRequests).Assembly.Location).Directory.FullName;
            return new TestServer(Microsoft.AspNetCore.WebHost
                .CreateDefaultBuilder()
                .ConfigureAppConfiguration((builderContext, config) =>
                {
                    var builder = config
                        .SetBasePath(functionPath)
                        .AddJsonFile("appsettings.json", optional: false, reloadOnChange: true)
                        .AddJsonFile($"appsettings.{builderContext.HostingEnvironment.EnvironmentName}.json", optional: true, reloadOnChange: true)
                        .AddEnvironmentVariables();
                })
                .UseStartup<Startup>()
                .UseContentRoot(functionPath));
        }
        
        [FunctionName(nameof(HttpRequests))]
        public static async Task<HttpResponseMessage> Run(CancellationToken ct,
            [HttpTrigger(AuthorizationLevel.Anonymous, "get", "post", "put", "patch", "options", Route = "{*x:regex(^(?!admin|debug|monitoring).*$)}")]HttpRequestMessage req, TraceWriter log)
        {
            log.Info("C# HTTP trigger function processed a request.");
            var client = InternalTestServer.CreateClient();
            var responseMessage = await client.SendAsync(req, ct);
            return null;
        }
    }

    public class Startup
    {       
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddMvc();
        }

        public void Configure(IApplicationBuilder app)
        {
            app.UseMvc();
        }
    }

    [Route("api/[controller]")]
    public class TodoController : Controller
    {
        private readonly IConfiguration _configuration;

        public TodoController(IConfiguration configuration)
        {
            _configuration = configuration;
        }

        [HttpGet("{id}", Name = "GetTodo")]
        public IActionResult GetById(long id)
        {
            return new ObjectResult(id + _configuration["Message"]);
        }
    }
}

WebApiOverAzureFunctions.csproj

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>
    <AzureFunctionsVersion>v2</AzureFunctionsVersion>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="Microsoft.AspNetCore" Version="2.0.1" />
    <PackageReference Include="Microsoft.AspNetCore.Mvc" Version="2.0.2" />
    <PackageReference Include="Microsoft.AspNetCore.Mvc.ViewFeatures" Version="2.0.2" />
    <PackageReference Include="Microsoft.AspNetCore.TestHost" Version="2.0.1" />
    <PackageReference Include="Microsoft.NET.Sdk.Functions" Version="1.0.6" />
  </ItemGroup>
  <ItemGroup>
    <None Update="host.json">
      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
    </None>
    <None Update="local.settings.json">
      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
      <CopyToPublishDirectory>Never</CopyToPublishDirectory>
    </None>
  </ItemGroup>
</Project>

Please note that the goal of this example is to show a concept where a WebAPI app would be hosted inside and Azure Functions. This example was not tested for performance and rely on test related classes which were not meant to be used this way. Also note that while this code compile, you get an assembly loading runtime error when executing the application.

Also worth checking out https://github.com/tntwist/NL.Serverless.AspNetCore which is a framework which achieves the same thing and seems to work well with azure functions v3.