ASP.NET MVC ValueProvider 體系第三篇：ValueProviderFactory 與註冊機制

ValueProviderFactory 抽象基類

public abstract class ValueProviderFactory
{
    public abstract IValueProvider GetValueProvider(ControllerContext controllerContext);
}

六大內建 Factory

ValueProviderFactories 靜態註冊中心

public static class ValueProviderFactories
{
    public static ValueProviderFactoryCollection Factories { get; }
}

public class ValueProviderFactoryCollection : Collection<ValueProviderFactory>
{
    public IValueProvider GetValueProvider(ControllerContext controllerContext)
    {
        var providers = new List<IValueProvider>();
        foreach (var factory in this)
            providers.Add(factory.GetValueProvider(controllerContext));
        return new ValueProviderCollection(providers);
    }
}

預設註冊順序與優先級

1. ChildActionValueProviderFactory
2. FormValueProviderFactory
3. JsonValueProviderFactory
4. RouteDataValueProviderFactory
5. QueryStringValueProviderFactory
6. HttpFileCollectionValueProviderFactory

優先級規則：集合中越前面的 Provider 產生的 ValueProvider 越優先被 ValueProviderCollection 查詢。同名鍵存在於表單與查詢字串時，表單值勝出。

自訂 Factory 範例：CookieValueProviderFactory

public class CookieValueProviderFactory : ValueProviderFactory
{
    public override IValueProvider GetValueProvider(ControllerContext controllerContext)
    {
        var cookies = controllerContext.HttpContext.Request.Cookies;
        var dict = cookies.AllKeys.ToDictionary(k => k, k => (object)cookies[k].Value);
        return new DictionaryValueProvider<object>(dict, CultureInfo.InvariantCulture);
    }
}

// 註冊於 Global.asax
ValueProviderFactories.Factories.Add(new CookieValueProviderFactory());
// 或插入特定位置調整優先級
ValueProviderFactories.Factories.Insert(2, new CookieValueProviderFactory());

UML 關係圖

三層架構總結

• ValueProvider：實際資料存取介面 (ContainsPrefix, GetValue)。
• ValueProviderFactory：建立 ValueProvider 的工廠，隔離建構邏輯。
• ValueProviderFactories：全域 Factory 集合，決定優先級順序。

總結

ValueProvider 體系採工廠模式分離建構與使用，六大內建 Factory 涵蓋主流資料來源。自訂 Factory 可輕鬆擴充新資料來源（Cookie、Header、Session 等），透過集合順序控制優先級，完美符合開放封閉原則。