Program.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Azure.KeyVault;
using Microsoft.Azure.Services.AppAuthentication;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.Configuration.AzureKeyVault;
using Microsoft.Extensions.Logging;

namespace aspnetcore_msi_keyvault
{
    public class Program
    {
        public static void Main(string[] args)
        {
            BuildWebHost(args).Run();
        }

        public static IWebHost BuildWebHost(string[] args) =>
            WebHost.CreateDefaultBuilder(args)
                .ConfigureAppConfiguration((ctx, builder) =>
                {
                    var keyVaultEndpoint = GetKeyVaultEndpoint();
                    if (!string.IsNullOrEmpty(keyVaultEndpoint))
                    {
                        var azureServiceTokenProvider = new AzureServiceTokenProvider();
                        var keyVaultClient = new KeyVaultClient(new KeyVaultClient.AuthenticationCallback(azureServiceTokenProvider.KeyVaultTokenCallback));
                        builder.AddAzureKeyVault(keyVaultEndpoint, keyVaultClient, new DefaultKeyVaultSecretManager());
                    }
                })
                .UseStartup<Startup>()
                .Build();

        private static string GetKeyVaultEndpoint() => Environment.GetEnvironmentVariable("KEYVAULT_ENDPOINT");
    }
}