ASP.NET MVC ValueProvider 體系第二篇：DictionaryValueProvider、RouteDataValueProvider 與 HttpFileCollectionValueProvider

DictionaryValueProvider<TValue>：泛型字典值提供者

public class DictionaryValueProvider<TValue> : IEnumerableValueProvider, IValueProvider
{
    public DictionaryValueProvider(IDictionary<string, TValue> dictionary, CultureInfo culture);
    public virtual bool ContainsPrefix(string prefix);
    public virtual IDictionary<string, string> GetKeysFromPrefix(string prefix);
    public virtual ValueProviderResult GetValue(string key);
}

與 NameValueCollectionValueProvider 邏輯相似，差異在資料來源為強型別 IDictionary<string, TValue>，鍵唯一。

RouteDataValueProvider：路由資料值提供者

public sealed class RouteDataValueProvider : DictionaryValueProvider<object>
{
    public RouteDataValueProvider(ControllerContext controllerContext)
        : base(controllerContext.RouteData.Values, CultureInfo.InvariantCulture) { }
}

直接包裝 RouteData.Values (RouteValueDictionary)，提供路由參數（controller、action、id 等）繫結。

HttpFileCollectionValueProvider：上傳檔案值提供者

HTTP 檔案集合結構

public abstract class HttpRequestBase
{
    public virtual HttpFileCollectionBase Files { get; }
}

public abstract class HttpFileCollectionBase : NameObjectCollectionBase, ICollection, IEnumerable
{
    public virtual string[] AllKeys { get; }
    public virtual HttpPostedFileBase this[int index] { get; }
    public virtual HttpPostedFileBase this[string name] { get; }
}

public abstract class HttpPostedFileBase
{
    public virtual void SaveAs(string filename);
    public virtual int ContentLength { get; }
    public virtual string ContentType { get; }
    public virtual string FileName { get; }
    public virtual Stream InputStream { get; }
}

Provider 實作

public sealed class HttpFileCollectionValueProvider : DictionaryValueProvider<HttpPostedFileBase[]>
{
    public HttpFileCollectionValueProvider(ControllerContext controllerContext);
}

將 Request.Files 轉為 Dictionary<string, HttpPostedFileBase[]>，鍵為 input name，值為同名檔案陣列。支援多檔上傳繫結至 HttpPostedFileBase[] 或 IEnumerable<HttpPostedFileBase> 參數。

表單對應範例

<input name="Avatar" type="file" />
<input name="Avatar" type="file" />
<input name="Documents" type="file" />
<input name="Documents" type="file" />
<input name="Documents" type="file" />

Action：

public ActionResult Upload(HttpPostedFileBase[] Avatar, List<HttpPostedFileBase> Documents) { ... }

Avatar 繫結 2 個檔案陣列，Documents 繫結 3 個檔案列表。

總結

DictionaryValueProvider 為字典來源通用基類，RouteDataValueProvider 與 HttpFileCollectionValueProvider 分別包裝路由資料與上傳檔案。理解其繼承關係與資料轉換邏輯，可自訂 Provider 整合新型字典資料來源。