Azure函数应用中在Startup类外调用IConfigurationBuilder.Build()是否有问题？

我们在Azure函数应用项目中使用Azure App Configuration，为了给开发者提供顺畅的体验，仅在local.settings.json中保留连接字符串和调试客户端配置，示例如下：

{
  "IsEncrypted": false,
  "Values": {
    "AzureFunctionsJobHost__logging__logLevel__Evn": "Trace",
    "FUNCTIONS_WORKER_RUNTIME": "dotnet",
    "AzureWebJobsStorage": "",
    "AppConfig": "Endpoint=***;Id=***",
    "Client": "clientId",
  }
}

核心扩展方法实现 #

为了让local.settings.json的配置优先于Azure App Configuration（方便开发者将部分配置指向本地数据库、存储账户等），我们实现了以下扩展方法：

AddAppConfiguration 扩展方法 #

public static void AddAppConfiguration(this IConfigurationBuilder configurationBuilder,
    string client = null)
{
    if (!string.IsNullOrWhiteSpace(Environment.GetEnvironmentVariable("AppConfig")))
    {
        configurationBuilder.AddAzureAppConfiguration(options =>
        {
            options.Connect(Environment.GetEnvironmentVariable("AppConfig"))
                .Select(KeyFilter.Any, LabelFilter.Null)
                .Select(KeyFilter.Any, client ?? Environment.GetEnvironmentVariable("Client"));
        });

        configurationBuilder.PrioritizeAppSettings(); 
        configurationBuilder.SetEnvironmentVariableFromAzureIfNotExists("AzureWebJobsStorage");
    }
}

PrioritizeAppSettings 扩展方法 #

private static void PrioritizeAppSettings(this IConfigurationBuilder builder)
{
    // appsettings.json
    var appsettingsJson = builder.Sources.Where(x => x.GetType() == typeof(JsonConfigurationSource)).ToList();
    foreach (var source in appsettingsJson)
    {
        builder.Sources.Remove(source);
        builder.Sources.Add(source);
    }

    // local.settings.json
    var environmentJson = builder.Sources.Where(x => x.GetType() == typeof(EnvironmentVariablesConfigurationSource)).ToList();
    foreach (var source in environmentJson)
    {
        builder.Sources.Remove(source);
        builder.Sources.Add(source);
    }
}

在Startup中的调用 #

class Startup : FunctionsStartup
{
    public override void ConfigureAppConfiguration(IFunctionsConfigurationBuilder builder)
    {
        builder.ConfigurationBuilder.AddAppConfiguration();
    }

    public override void Configure(IFunctionsHostBuilder builder)
    {     
        // add services      
    }
}

问题与解决方案 #

我们遇到的问题是：AzureWebJobsStorage配置值为空字符串时，希望自动从Azure App Configuration中获取对应值。为此实现了以下方法：

private static void SetEnvironmentVariableFromAzureIfNotExists(this IConfigurationBuilder builder, string key)
{
    if (string.IsNullOrWhiteSpace(Environment.GetEnvironmentVariable(key)))
    {
        builder.Build().Providers
            .FirstOrDefault(p => p.GetType().Name == "AzureAppConfigurationProvider")
            .TryGet(key, out var constring);

        if (!string.IsNullOrWhiteSpace(constring))
            Environment.SetEnvironmentVariable(key, constring);
    }
}

该方法在AddAppConfiguration()中PrioritizeAppSettings()之后调用，目前本地运行正常，但存在疑问：在Startup类外调用builder.Build()是否存在问题？ 因为.NET底层似乎也会执行类似的构建操作。

内容的提问来源于stack exchange，提问作者Clayton Treder