熟悉ASP.NET的開發者一定對web.config文件不陌生。在ASP.NET環境中，要想添加配置參數，一般也都會在此文件中操作。其中最常用的莫過于AppSettings與ConnectionStrings兩項。而要在代碼中獲得文件中的配置信息，ConfigurationManager則是必不可少需要引入的程序集。

然而到了ASP.NET Core時代，存儲與讀取配置的方式都發生了改變。

如果對ASP.NET Core項目有所了解的話，應該會看到過appsettings.json這個文件。這里就從JSON文件配置方式開始解釋ASP.NET Core中是如何讀取配置信息的。

假設有預先設置的appsettings.json文件：

{ "option1": "value1_from_json", "option2": 2, "subsection": {  "suboption1": "subvalue1_from_json" }, "wizards": [  {   "Name": "Gandalf",   "Age": "1000"  },  {   "Name": "Harry",   "Age": "17"  } ]}

在代碼中讀取可以按下面的方式操作：

public class Program{  public static IConfiguration Configuration { get; set; }  public static void Main(string[] args = null)  {    var builder = new ConfigurationBuilder()      .SetBasePath(Directory.GetCurrentDirectory())      .AddJsonFile("appsettings.json");    Configuration = builder.Build();    Console.WriteLine($"option1 = {Configuration["Option1"]}");    Console.WriteLine($"option2 = {Configuration["option2"]}");    Console.WriteLine(      $"suboption1 = {Configuration["subsection:suboption1"]}");    Console.WriteLine();    Console.WriteLine("Wizards:");    Console.Write($"{Configuration["wizards:0:Name"]}, ");    Console.WriteLine($"age {Configuration["wizards:0:Age"]}");    Console.Write($"{Configuration["wizards:1:Name"]}, ");    Console.WriteLine($"age {Configuration["wizards:1:Age"]}");    Console.WriteLine();    Console.WriteLine("Press a key...");    Console.ReadKey();  }}

首先，實例化一個ConfigurationBuilder對象，接著設置基礎路徑。

SetBasePath的操作其實是在ConfigurationBuilder的屬性字典里設置FileProvider的值。public static IConfigurationBuilder SetBasePath(this IConfigurationBuilder builder, string basePath){  ...    return builder.SetFileProvider(new PhysicalFileProvider(basePath));}public static IConfigurationBuilder SetFileProvider(this IConfigurationBuilder builder, IFileProvider fileProvider){  ...  builder.Properties[FileProviderKey] = fileProvider ?? throw new ArgumentNullException(nameof(fileProvider));  return builder;}

然后是添加JSON文件。

public static IConfigurationBuilder AddJsonFile(this IConfigurationBuilder builder, IFileProvider provider, string path, bool optional, bool reloadOnChange){  ...  return builder.AddJsonFile(s =>  {    s.FileProvider = provider;    s.Path = path;    s.Optional = optional;    s.ReloadOnChange = reloadOnChange;    s.ResolveFileProvider();  });}public static IConfigurationBuilder AddJsonFile(this IConfigurationBuilder builder, Action<JsonConfigurationSource> configureSource)  => builder.Add(configureSource);