问题：
问题出在System.Web.Mvc 命名空间中的JsonValueProviderFactory 类中。实际上，如果你反编译System.Web.Mvc.dll 并找到JsonValueProviderFactory 类，你会看到在GetDeserializedObject 方法中它使用了JavaScriptSerializer，而没有为MaxJsonLength 设置任何值：
private static object GetDeserializedObject(ControllerContext controllerContext)
{
if (!controllerContext.HttpContext.Request.ContentType.StartsWith("application/json", StringComparison.OrdinalIgnoreCase))
{
return null;
}
StreamReader streamReader = new StreamReader(controllerContext.HttpContext.Request.InputStream);
string text = streamReader.ReadToEnd();
if (string.IsNullOrEmpty(text))
{
return null;
}
// The problem is here, not given. javaScriptSerializer.MaxJsonLength The default value is 2097152 bytes, that is 2. M
JavaScriptSerializer javaScriptSerializer = new JavaScriptSerializer();
return javaScriptSerializer.DeserializeObject(text);
}
解决方案：
您可以重写JsonValueProviderFactory 类并设置javaScriptSerializer.MaxJsonLength，然后在Global.asax 中的Application_Start() 方法中替换该类，如下所示：
ValueProviderFactories.Factories.Remove(ValueProviderFactories.Factories.OfType<JsonValueProviderFactory>().FirstOrDefault());
ValueProviderFactories.Factories.Add(new MyJsonValueProviderFactory());
这是完整的工作代码：
using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Configuration;
using System.Globalization;
using System.IO;
using System.Web.Mvc;
using System.Web.Mvc.Properties;
using System.Web.Script.Serialization;
namespace XXX
{
public sealed class MyJsonValueProviderFactory : ValueProviderFactory
{
private class EntryLimitedDictionary
{
private static int _maximumDepth = GetMaximumDepth();
private readonly IDictionary<string, object> _innerDictionary;
private int _itemCount;
public EntryLimitedDictionary(IDictionary<string, object> innerDictionary)
{
this._innerDictionary = innerDictionary;
}
public void Add(string key, object value)
{
if (++this._itemCount > _maximumDepth)
{
//throw new InvalidOperationException(MvcResources.JsonValueProviderFactory_RequestTooLarge);
throw new InvalidOperationException("itemCount is over maximumDepth");
}
this._innerDictionary.Add(key, value);
}
private static int GetMaximumDepth()
{
NameValueCollection appSettings = ConfigurationManager.AppSettings;
if (appSettings != null)
{
string[] values = appSettings.GetValues("aspnet:MaxJsonDeserializerMembers");
int result;
if (values != null && values.Length > 0 && int.TryParse(values[0], out result))
{
return result;
}
}
return 1000;
}
}
private static void AddToBackingStore(EntryLimitedDictionary backingStore, string prefix, object value)
{
IDictionary<string, object> dictionary = value as IDictionary<string, object>;
if (dictionary != null)
{
foreach (KeyValuePair<string, object> current in dictionary)
{
AddToBackingStore(backingStore, MakePropertyKey(prefix, current.Key), current.Value);
}
return;
}
IList list = value as IList;
if (list != null)
{
for (int i = 0; i < list.Count; i++)
{
AddToBackingStore(backingStore, MakeArrayKey(prefix, i), list[i]);
}
return;
}
backingStore.Add(prefix, value);
}
private static object GetDeserializedObject(ControllerContext controllerContext)
{
if (!controllerContext.HttpContext.Request.ContentType.StartsWith("application/json", StringComparison.OrdinalIgnoreCase))
{
return null;
}
StreamReader streamReader = new StreamReader(controllerContext.HttpContext.Request.InputStream);
string text = streamReader.ReadToEnd();
if (string.IsNullOrEmpty(text))
{
return null;
}
JavaScriptSerializer javaScriptSerializer = new JavaScriptSerializer();
// To solve this problem:
javaScriptSerializer.MaxJsonLength = int.MaxValue;
// ----------------------------------------
return javaScriptSerializer.DeserializeObject(text);
}
public override IValueProvider GetValueProvider(ControllerContext controllerContext)
{
if (controllerContext == null)
{
throw new ArgumentNullException("controllerContext");
}
object deserializedObject = GetDeserializedObject(controllerContext);
if (deserializedObject == null)
{
return null;
}
Dictionary<string, object> dictionary = new Dictionary<string, object>(StringComparer.OrdinalIgnoreCase);
EntryLimitedDictionary backingStore = new EntryLimitedDictionary(dictionary);
AddToBackingStore(backingStore, string.Empty, deserializedObject);
return new DictionaryValueProvider<object>(dictionary, CultureInfo.CurrentCulture);
}
private static string MakeArrayKey(string prefix, int index)
{
return prefix + "[" + index.ToString(CultureInfo.InvariantCulture) + "]";
}
private static string MakePropertyKey(string prefix, string propertyName)
{
if (!string.IsNullOrEmpty(prefix))
{
return prefix + "." + propertyName;
}
return propertyName;
}
}
}
参考资料：
https://www.fatalerrors.org/a/net-mvc-json-javascriptserializer-string-exceeds-the-maxjsonlength.html