JsonValueProvider 从 MVC 应用程序抛出错误

我们开始在日志中看到这一点，知道从哪里开始寻找问题吗？调用堆栈中没有任何内容可以帮助我们开始。

    Message :An item with the same key has already been added. at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource) at 
System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean 
add) at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value) at 
System.Web.Mvc.JsonValueProviderFactory.EntryLimitedDictionary.Add(String key, 
Object value) at 
System.Web.Mvc.JsonValueProviderFactory.AddToBackingStore(EntryLimitedDictionary
backingStore, String prefix, Object value) at 
System.Web.Mvc.JsonValueProviderFactory.AddToBackingStore(EntryLimitedDictionary
backingStore, String prefix, Object value) at 
System.Web.Mvc.JsonValueProviderFactory.AddToBackingStore(EntryLimitedDictionary backingStore, String prefix, Object value) at 
System.Web.Mvc.JsonValueProviderFactory.AddToBackingStore(EntryLimitedDictionary backingStore, String prefix, Object value) at 
System.Web.Mvc.JsonValueProviderFactory.AddToBackingStore(EntryLimitedDictionary backingStore, String prefix, Object value) at 
System.Web.Mvc.JsonValueProviderFactory.AddToBackingStore(EntryLimitedDictionary backingStore, String prefix, Object value) at 
System.Web.Mvc.JsonValueProviderFactory.AddToBackingStore(EntryLimitedDictionary backingStore, String prefix, Object value) at 
System.Web.Mvc.JsonValueProviderFactory.GetValueProvider(ControllerContext controllerContext) at System.Web.Mvc.ValueProviderFactoryCollection.<>c__DisplayClassc.<GetValueProvider>b__7(ValueProviderFactory factory) at 
System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext() at 
System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext() at 
System.Collections.Generic.List`1..ctor(IEnumerable`1 collection) at 
System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source) at 
System.Web.Mvc.ValueProviderFactoryCollection.GetValueProvider(ControllerContext controllerContext) at System.Web.Mvc.ControllerBase.get_ValueProvider() at 
System.Web.Mvc.ControllerActionInvoker.GetParameterValue(ControllerContext controllerContext, ParameterDescriptor parameterDescriptor) at 
System.Web.Mvc.ControllerActionInvoker.GetParameterValues(ControllerContext controllerContext, ActionDescriptor actionDescriptor) at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass25.
<BeginInvokeAction>b__1e(AsyncCallback asyncCallback, Object asyncState) at 
System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResult`1.Begin(AsyncCallback callback, Object state, Int32 timeout) at 
System.Web.Mvc.Async.AsyncControllerActionInvoker.BeginInvokeAction(ControllerContext controllerContext, String actionName, AsyncCallback callback, Object 
state) at System.Web.Mvc.Controller.<>c__DisplayClass1d.<BeginExecuteCore>b__17(AsyncCallback asyncCallback, Object asyncState) at 
System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResult`1.Begin(AsyncCallback callback, Object state, Int32 timeout) at 
System.Web.Mvc.Controller.BeginExecuteCore(AsyncCallback callback, Object state) at 
System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResult`1.Begin(AsyncCallback callback, Object state, Int32 timeout) at 
System.Web.Mvc.Controller.BeginExecute(RequestContext requestContext, AsyncCallback callback, Object state) at 
System.Web.Mvc.Controller.System.Web.Mvc.Async.IAsyncController.BeginExecute(RequestContext requestContext, AsyncCallback callback, Object state) at 
System.Web.Mvc.MvcHandler.<>c__DisplayClass8.<BeginProcessRequest>b__2(AsyncCallback asyncCallback, Object asyncState) at 
System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResult`1.Begin(AsyncCallback callback, Object state, Int32 timeout) at 
System.Web.Mvc.MvcHandler.BeginProcessRequest(HttpContextBase httpContext, AsyncCallback callback, Object state) at 
System.Web.Mvc.MvcHandler.BeginProcessRequest(HttpContext httpContext, AsyncCallback callback, Object state) at 
System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.BeginProcessRequest(HttpContext context, AsyncCallback cb, Object extraData) at 
System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

当你的控制器方法接收到 json 时，DefaultModelBinder使用JsonValueProviderFactory，它使用JavaScriptSerializer反序列化 json 字符串并将名称/值对添加到传递给的字典中DictionaryValueProvider这反过来又提供了绑定的值。

您的错误是因为您的 json 包含一个具有重复属性名称的对象而引起的，例如{ name: someValue, NAME: anotherValue }那么一次name: someValue已添加到字典中，添加NAME: anotherValue由于重复（不区分大小写）键而引发异常。这是有道理的，因为一个模型不能包含两个同名的属性。

虽然最好的解决方案是追踪并纠正问题的来源'无效的'json，你可以自己写ValueProvideFactory仅绑定第一个值并忽略具有相同键的后续值。要避免重新发明轮子，请使用以下步骤：

1. 复制源代码JsonValueProviderFactory.cs 源 代码 https://aspnetwebstack.codeplex.com/SourceControl/latest#src/System.Web.Mvc/JsonValueProviderFactory.cs进入你的项目
2. 重命名命名空间（例如yourProject.ValueProviderFactories） 和 添加一个using System.Web.Mvc;陈述
3. 重命名该类（例如InvalidJsonValueProviderFactory)

4. 修改里面的以下代码private class EntryLimitedDictionary

   public void Add(string key, object value)
   {
       if (++_itemCount > _maximumDepth)
       {
           // throw new InvalidOperationException(MvcResources.JsonValueProviderFactory_RequestTooLarge);
       }
       // Add the following if block
       if (_innerDictionary.ContainsKey(key))
       {
           return;
       }
       _innerDictionary.Add(key, value);
   }
   

5. 将以下内容添加到您的global.asax.cs file

   ValueProviderFactories.Factories.Insert(0, new yourProject.ValueProviderFactories.InvalidJsonValueProviderFactory());
   

请注意，需要先添加它，因为JsonValueProviderFactory已添加到工厂集合中。

这将阻止您的异常，但当然意味着重复的属性将被丢弃。如果获得重复项很重要，那么您可以考虑进行更多修改，以使用包含索引器的不同唯一键创建新的字典条目，例如

private int _index = 0;
....
if (_innerDictionary.ContainsKey(key))
{
    string invalidKey = string.Format("duplicates[{0}]", _index++);
    string invalidValue = string.Format("{0}|{1}", key, value);
    _innerDictionary.Add(invalidKey, invalidValue);
    return;
}

where index将从零开始并每次递增。然后你可以在方法中添加一个额外的参数string[] duplicates其中将填充重复值。至少它有助于调试问题的根源。