通過實例模擬ASP.NET MVC的Model綁定的機制：集合+字典

在本系列的前面兩篇文章（《簡單類型+復雜類型》、《數組》）我們通過創建的實例程序模擬了ASP.NET MVC默認使用的DefaultModelBinder對簡單類型、復雜類型以及數組對象的Model綁定。現在我們按照相同的方式來分析基於集合和字典類型的Model綁定是如何實現的。[源代碼從這裡下載]

一、集合

這裡的集合指的是除數組和字典之外的所有實現IEnumerable<T>接口的類型。和基於數組的Model綁定類似，ValueProvider可以將多個同名的數據項作為集合的元素，基於索引（基零整數和字符串）的數據項命名方式同樣適用。我們對自定義的DefaultModelBinder作了如下的完善使之支持集合類型的Model綁定。

   1: public class DefaultModelBinder
   2: {
   3:     //其他成員
   4:     public object BindModel(Type parameterType, string prefix)
   5:     {
   6:         if (!this.ValueProvider.ContainsPrefix(prefix))
   7:         {
   8:             return null;
   9:         }
  10:         ModelMetadata modelMetadata = ModelMetadataProviders.Current.GetMetadataForType(() => null, parameterType);
  11:         if (!modelMetadata.IsComplexType)
  12:         {
  13:             return this.ValueProvider.GetValue(prefix).ConvertTo(parameterType);
  14:         }
  15:         if (parameterType.IsArray)
  16:         {
  17:             return BindArrayModel(parameterType, prefix);
  18:         }
  19:         object model = CreateModel(parameterType);
  20:         Type enumerableType = ExtractGenericInterface(parameterType, typeof(IEnumerable<>));
  21:         if (null != enumerableType)
  22:         {
  23:             return BindCollectionModel(prefix, model, enumerableType);
  24:         }
  25:         foreach (PropertyDescriptor property in TypeDescriptor.GetProperties(parameterType))
  26:         {
  27:             string key = prefix == "" ? property.Name : prefix + "." + property.Name;
  28:             property.SetValue(model, BindModel(property.PropertyType, key));
  29:         }
  30:         return model;
  31:     }
  32:
  33:     private object BindCollectionModel(string prefix, object model, Type enumerableType)
  34:     {
  35:         List<object> list = new List<object>();
  36:         bool numericIndex;
  37:         IEnumerable<string> indexes = GetIndexes(prefix, out numericIndex);
  38:         Type elementType = enumerableType.GetGenericArguments()[0];
  39:
  40:         if (!string.IsNullOrEmpty(prefix) && this.ValueProvider.ContainsPrefix(prefix))
  41:         {
  42:             IEnumerable enumerable = this.ValueProvider.GetValue(prefix).ConvertTo(enumerableType) as IEnumerable;
  43:             if (null != enumerable)
  44:             {
  45:                 foreach (var value in enumerable)
  46:                 {
  47:                     list.Add(value);
  48:                 }
  49:             }
  50:         }
  51:         foreach (var index in indexes)
  52:         {
  53:             string indexPrefix = prefix + "[" + index + "]";
  54:             if (!this.ValueProvider.ContainsPrefix(indexPrefix) && numericIndex)
  55:             {
  56:                 break;
  57:             }
  58:             list.Add(BindModel(elementType, indexPrefix));
  59:         }
  60:         if (list.Count == 0)
  61:         {
  62:             return null;
  63:         }
  64:         ReplaceHelper.ReplaceCollection(elementType, model, list);
  65:         return model;
  66:     }
  67:
  68:     private Type ExtractGenericInterface(Type queryType, Type interfaceType)
  69:     {
  70:         Func<Type, bool> predicate = t => t.IsGenericType && (t.GetGenericTypeDefinition() == interfaceType);
  71:         if (!predicate(queryType))
  72:         {
  73:             return queryType.GetInterfaces().FirstOrDefault<Type>(predicate);
  74:         }
  75:         return queryType;
  76:     }
  77: }

如上面的代碼片斷所示，在BindModel方法中我們通過調用ExtractGenericInterface判斷目標類型是否實現了IEnumerable<T>接口，如果實現了該接口則提取泛型元素類型。針對集合的Model綁定實現在方法BindCollectionModel中，我們按照數組綁定的方式得的針對目標集合對象的所有元素對象，並將其添加到一個List<object>對象中，然後調用ReplaceHelper 的靜態方法ReplaceCollection將該列表中的元素拷貝到預先創建的Model對象中。定義在ReplaceHelper的靜態方法ReplaceCollection定義如下：

   1: internal static class ReplaceHelper
   2: {
   3:     private static MethodInfo replaceCollectionMethod = typeof(ReplaceHelper).GetMethod("ReplaceCollectionImpl", BindingFlags.Static |BindingFlags.NonPublic);
   4:
   5:      public static void ReplaceCollection(Type collectionType, object collection, object newContents)
   6:     {
   7:         replaceCollectionMethod.MakeGenericMethod(new Type[] { collectionType }).Invoke(null, new object[] { collection, newContents });
   8:     }
   9:     private static void ReplaceCollectionImpl<T>(ICollection<T> collection, IEnumerable newContents)
  10:     {
  11:         collection.Clear();
  12:         if (newContents != null)
  13:         {
  14:             foreach (object obj2 in newContents)
  15:             {
  16:                 T item = (obj2 is T) ? ((T)obj2) : default(T);
  17:                 collection.Add(item);
  18:             }
  19:         }
  20:     }
  21: }