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[ArgumentException: 使用 JSON JavaScriptSerializer 序列化或还原序列化期间发生错误。字符串的长度超过在 maxJsonLength 属性上设定的值。
参数名称: input]
   System.Web.Script.Serialization.JavaScriptSerializer.Deserialize(JavaScriptSerializer serializer, String input, Type type, Int32 depthLimit) +168
   System.Web.Mvc.JsonValueProviderFactory.GetDeserializedObject(ControllerContext controllerContext) +213
   System.Web.Mvc.JsonValueProviderFactory.GetValueProvider(ControllerContext controllerContext) +16
   System.Web.Mvc.ValueProviderFactoryCollection.GetValueProvider(ControllerContext controllerContext) +69
   System.Web.Mvc.ControllerBase.get_ValueProvider() +30　　

由于前端 Post 到 Action 的参数太大，超过了2M，还没进入后台的 Action 方法就报错了。这个问题困扰了很久，一直未解决。网上找了几个方法都无效。

在 web.config 中加入这些，没有作用：

<appSettings>
  <add key="aspnet:MaxJsonDeserializerMembers" value="2147483647" />
  <add key="aspnet:UpdatePanelMaxScriptLength" value="2147483647" />
</appSettings>

在 web.config 中加入这些，也没有作用：

<system.web.extensions>
  <scripting>
    <webServices>
      <jsonSerialization maxJsonLength="2147483647">
      </jsonSerialization>
    </webServices>
  </scripting>
</system.web.extensions>

仔细看了一下异常信息，发现，是在System.Web.Mvc.JsonValueProviderFactory 里调用的 JavaScriptSerializer：

于是查了一下 ，发现 JsonValueProviderFactory 在 System.Web.Mvc.dll 程序集里的：

反编译 System.Web.Mvc.dll 找到 JsonValueProviderFactory 类：

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.Properties;
using System.Web.Script.Serialization;
namespace System.Web.Mvc
{
	public sealed class JsonValueProviderFactory : ValueProviderFactory
	{
		private class EntryLimitedDictionary
		{
			private static int _maximumDepth = JsonValueProviderFactory.EntryLimitedDictionary.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 > JsonValueProviderFactory.EntryLimitedDictionary._maximumDepth)
				{
					throw new InvalidOperationException(MvcResources.JsonValueProviderFactory_RequestTooLarge);
				}
				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(JsonValueProviderFactory.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)
				{
					JsonValueProviderFactory.AddToBackingStore(backingStore, JsonValueProviderFactory.MakePropertyKey(prefix, current.Key), current.Value);
				}
				return;
			}
			IList list = value as IList;
			if (list != null)
			{
				for (int i = 0; i < list.Count; i++)
				{
					JsonValueProviderFactory.AddToBackingStore(backingStore, JsonValueProviderFactory.MakeArrayKey(prefix, i), list);
				}
				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;
			}
			// 问题就出在这里，没有给 javaScriptSerializer.MaxJsonLength 赋值，其默认值是 2097152 字节，即2M
			JavaScriptSerializer javaScriptSerializer = new JavaScriptSerializer();
			return javaScriptSerializer.DeserializeObject(text);
		}
		public override IValueProvider GetValueProvider(ControllerContext controllerContext)
		{
			if (controllerContext == null)
			{
				throw new ArgumentNullException("controllerContext");
			}
			object deserializedObject = JsonValueProviderFactory.GetDeserializedObject(controllerContext);
			if (deserializedObject == null)
			{
				return null;
			}
			Dictionary<string, object> dictionary = new Dictionary<string, object>(StringComparer.OrdinalIgnoreCase);
			JsonValueProviderFactory.EntryLimitedDictionary backingStore = new JsonValueProviderFactory.EntryLimitedDictionary(dictionary);
			JsonValueProviderFactory.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;
		}
	}
}

在 JavaScriptSerializer 没有设置 MaxJsonLength，默认值是 2097152 字节，即2M。　

解决此问题的方法就是 把 javaScriptSerializer.MaxJsonLength = int.MaxValue; （int.MaxValue 值是 2147483647 字节，即2048M）

自己重写类 JsonValueProviderFactory 命名为 MyJsonValueProviderFactory：

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);
                }
                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();
            // 解决这个问题：
            // 使用 JSON JavaScriptSerializer 序列化或还原序列化期间发生错误。字符串的长度超过在 maxJsonLength 属性上设定的值。
            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;
        }
    }
}

然后在 Global.asax 中的 Application_Start() 方法里，加入如下代码，用 MyJsonValueProviderFactory 类代替 System.Web.Mvc.dll 程序集中的 JsonValueProviderFactory 类。

ValueProviderFactories.Factories.Remove(ValueProviderFactories.Factories.OfType<JsonValueProviderFactory>().FirstOrDefault());
ValueProviderFactories.Factories.Add(new MyJsonValueProviderFactory());

 至此，.NET MVC 超出 maxJsonLength 的问题终于解决了！