这个组件的主要作用是将设置(Setttings)的内容，根据键值关系填充到已经存在的实体或静态类中

先来一个最简单的例子

有一个叫tests的项目，app.config的配置文件内容如下：

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <appSettings>
    <add key="tests.datetime" value="2017-1-1 0:0:0"/>
    <add key="tests.url" value="//baidu.com"/>
    <add key="tests.maxlength" value="9999.99"/>
    <add key="tests.minlength" value="0.01"/>
    <add key="tests.delay" value="1000"/>
    <add key="tests.enable" value="true"/>
  </appSettings>
</configuration>

* 在使用中我比较喜欢在设置前加上项目的前缀或命名空间, 以便区分不同项目间的同名设置项

过去在项目中使用配置文件一般是

static void Main(string[] args)
{
    var timeValue = System.Configuration.ConfigurationManager.AppSettings["tests.datetime"];
    if(!DateTime.TryParse(timeValue, out var time))
    {
        time = new DateTime(1970, 1, 1);
    }
    Console.WriteLine(time);
}

不仅臃肿，而且不易阅读和维护

好一点的做法是

static class Settings
{
    static Settings()
    {
        var settings = System.Configuration.ConfigurationManager.AppSettings;
        if (DateTime.TryParse(settings["tests.datetime"], out var time))
        {
            DateTime = new DateTime(1970, 1, 1);
        }
        if (Uri.TryCreate(settings["tests.url"],UriKind.RelativeOrAbsolute,out var url))
        {
            Url = url;
        }
        if (float.TryParse(settings["tests.maxlength"], out var maxlength))
        {
            MaxLength = maxlength;
        }
        if (float.TryParse(settings["tests.minlength"], out var minlength))
        {
            MinLength = minlength;
        }
        if (int.TryParse(settings["tests.delay"], out var delay))
        {
            Delay = delay;
        }
        if (bool.TryParse(settings["tests.enable"], out var enable))
        {
            Enable = enable;
        }
    }

    
    public static DateTime DateTime { get; private set; } = new DateTime(1970, 1, 1);
    public static Uri Url { get; private set; }
    public static float MaxLength { get; private set; }
    public static float MinLength { get; private set; }
    public static int Delay { get; private set; }
    public static bool Enable { get; private set; }
}

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine(Settings.DateTime);
        Console.WriteLine(Settings.Url);
        Console.WriteLine(Settings.MaxLength);
        Console.WriteLine(Settings.MinLength);
        Console.WriteLine(Settings.Delay);
        Console.WriteLine(Settings.Enable);
    }
}

这例子中是6个设置，而实际如果有10个甚至20个设置，那将变得非常枯燥

所以有了 SettingsFetcher

[SettingsGroupName("tests")]
static class Settings
{
    static Settings() => SettingsFetcher.Fill(typeof(Settings), false);

    public static DateTime DateTime { get; private set; } = new DateTime(1970, 1, 1);
    public static Uri Url { get; private set; }
    public static float MaxLength { get; private set; }
    public static float MinLength { get; private set; }
    public static int Delay { get; private set; }
    public static bool Enable { get; private set; }
}

使用 [SettingsGroupName("tests")] 这个特性来设置前缀，名称直接根据属性名称匹配
更多的用法可以参考