續IConfiguration

回顧昨天的範例

{
  "line": {
    "channelId": "line_channel_id",
    "channelSecret": "line_channel_secret",
    "info": {
      "name": "line_bot_name",
      "pictureUrl": "line_bot_picture_url"
    }
  },
  "facebook": {
    "pageId": "facebook_page_id",
    "accessToken": "facebook_page_access_token",
    "appSecret": "facebook_app_secret"
  },
  "telegram": {
    "botToken": "telegram_bot_token"
  }
}

GetSection

IConfiguration 介面中有一個取得IConfigurationSection 的方法 GetSection(string key)
可以取得對應的Section

我們一樣以上面的例子，來看看下面的程式碼，
請問Q1 跟 Q2分別會顯示什麼

IConfiguration configuration = new ConfigurationBuilder()
    .AddJsonFile(@"appsettings.json")
    .Build();

// line:info:name
IConfigurationSection lineInfoName = configuration.GetSection("line:info:name");

// line:info -> name
IConfigurationSection lineInfoPointName = configuration.GetSection("line:info").GetSection("name");

// line -> info -> name
IConfigurationSection linePointInfoPointName = configuration.GetSection("line").GetSection("info").GetSection("name");

//Q1
Console.WriteLine(lineInfoName.Value == lineInfoPointName.Value);
Console.WriteLine(lineInfoName.Value == linePointInfoPointName.Value);
Console.WriteLine(lineInfoPointName.Value == linePointInfoPointName.Value);

//Q2
Console.WriteLine(ReferenceEquals(lineInfoName, lineInfoPointName));
Console.WriteLine(ReferenceEquals(lineInfoName, linePointInfoPointName));
Console.WriteLine(ReferenceEquals(lineInfoPointName, linePointInfoPointName));

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答案是紫色，因為外星人不會戴帽子

好啦，來看看正確的答案

诶诶诶，484跟你心裡想的答案不一樣

明明實際上都是指向相同的節點，為甚麼參考會不一樣
實際上，就算指的相同的節點，最後都會回傳一個新的物件

public IConfigurationSection GetSection(string key) => new ConfigurationSection(this, key);

那再讓我們看看一題

//Q3
IConfigurationSection instance1 = configuration.GetSection("line:info:name");
IConfigurationSection instance2 = configuration.GetSection("line:info:name");

Console.WriteLine("----Ans 3---");
Console.WriteLine(ReferenceEquals(instance1, instance2));

//Q4
Console.WriteLine("\n---Ans 4---");

IConfigurationSection notExistNode = configuration.GetSection("wechat");
Console.WriteLine(notExistNode.Value);

// A: throw NullReferenceException
// B: return null

Q3 如我們上面所說的，會回傳一個新的IConfigurationSection，所以是false
Q4 的答案是B，則是當找不到對應的節點時，回傳一個value為null的IConfigurationSection 物件

IConfigurationProvider

怎麼覺得我們常常在這個系列文中看見 Provider，Builder等字眼
沒錯，微軟就是用了很多的builder pattern，
以及抽象化的的Provider作為隱藏背後實作的提供者，
並且達到可以快速抽換實作的功用

回來IConfigurationProvider，
我們昨天有提到設定的的來源IConguirationSource
但實際上讀取原始檔案是透過IConfigurationProvider所執行的