mvc源码解读(17)-数据绑定组件ModelBinder之ModelBinders

     在之前的文章中我们讲过，在获取ModelBinder的时候优先选择的是应用在Action参数上的CustomModelBinder，具体的实现是在静态类ModelBinders里面的GetBinderFromAttributesImpl方法：

private static IModelBinder GetBinderFromAttributesImpl(CustomModelBinderAttribute[] attrs, Func<string> errorMessageAccessor)

我们在分析源码的时候可以看到ModelBinder有一个Binders属性，是对ModelBinderDictionary类的一个封装，具体如下：

private static readonly ModelBinderDictionary _binders = CreateDefaultBinderDictionary();         public static ModelBinderDictionary Binders {get {return _binders;}}    private static ModelBinderDictionary CreateDefaultBinderDictionary() {                       // We can't add a binder to the HttpPostedFileBase type as an attribute, so we'll just                       // prepopulate the dictionary as a convenience to users.          ModelBinderDictionary binders = new ModelBinderDictionary() {                                       {typeof(HttpPostedFileBase), new HttpPostedFileBaseModelBinder() },                                       {typeof(byte[]), new ByteArrayModelBinder() },                                       {typeof(Binary), new LinqBinaryModelBinder() }                                     };                              return binders;         }

 mvc应用程序中提供了三种默认的ModelBinder供我们使用，我们逐一来看看。HttpPostedFileBaseModelBinder里面的成员如下：

public class HttpPostedFileBaseModelBinder : IModelBinder {         public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext) {                         HttpPostedFileBase theFile = controllerContext.HttpContext.Request.Files[bindingContext.ModelName];                         return ChooseFileOrNull(theFile);                }         // helper that returns the original file if there was content uploaded, null if empty                 internal static HttpPostedFileBase ChooseFileOrNull(HttpPostedFileBase rawFile) {                         // case 1: there was no <input type="file" ... /> element in the post                                 if (rawFile == null) {return null;}             // case 2: there was an <input type="file" ... /> element in the post, but it was left blank                             if (rawFile.ContentLength == 0 && String.IsNullOrEmpty(rawFile.FileName)) {return null;}             // case 3: the file was posted                           return rawFile;                 }  }

HttpPostedFileBaseModelBinder用于客户端上传文件时相关的绑定，看红色的英文注释，不解释。还有就是字节数组的绑定ByteArrayModelBinder，具体实现如下：

public class ByteArrayModelBinder : IModelBinder {                     public virtual object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext) {                       ValueProviderResult valueResult = bindingContext.ValueProvider.GetValue(bindingContext.ModelName);                        // case 1: there was no <input ... /> element containing this data                                   if (valueResult == null) {return null;}                       string value = valueResult.AttemptedValue;                       // case 2: there was an <input ... /> element but it was left blank                                   if (String.IsNullOrEmpty(value)) {return null;}                       // Future proofing. If the byte array is actually an instance of System.Data.Linq.Binary                                   // then we need to remove these quotes put in place by the ToString() method.                                   string realValue = value.Replace("\"", String.Empty);                                   return Convert.FromBase64String(realValue);                     }     }

还有就是LinqBinaryModelBinder的绑定，该类是继承了字节数组ByteArrayModelBinder类，具体的实现如下：

public class LinqBinaryModelBinder : ByteArrayModelBinder {                    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext) {                              byte[] byteValue = (byte[])base.BindModel(controllerContext, bindingContext);                               if (byteValue == null) {return null;}                   return new Binary(byteValue);          }     }

     我们之前介绍过mvc中获取ModelBinder的顺序，在Action方法的参数中没有标注CustomModelBinderAttribute和ModelBinderProviders中的BinderProviders中也没有提供ModelBinder的话，就会在全局文件中查找，如同ModelBinderProviders中的BinderProviders在Application_Start注册一样，ModelBinders提供的ModelBinder也是在全局文件的Application_Start注册：

protected void Application_Start()  {          ModelBinders.Binders.Add(***); }