Web.config jsonSerialization maxJsonLength ignored

Question

I have an MVC3 application running under .NET 4.0 and when I use JavascriptSerializer.Deserialize I'm getting an error.

Error during serialization or deserialization using the JSON JavaScriptSerializer. The length of the string exceeds the value set on the maxJsonLength property.

Reading Can I set an unlimited length for maxJsonLength in web.config I put the jsonSerialization maxJsonLength key in my web.config but it gets ignored. Though I can set the JavaScriptSerializer.MaxJsonLength property in code and it works fine.

I would like to have the value in the Web.config rather than code. Here is how I am using JavaScriptSerializer.

Dim client As New WebClient
...
Dim result = _client.DownloadString("Test")
Dim serializer = New JavaScriptSerializer
Return serializer.Deserialize(Of Foo)(result)

Here is my Web.config

<configuration>
    <configSections>
    ...
    </configSections>
    <appSettings>
    ...
    </appSettings>
    <system.web>
    <customErrors mode="On">
      <error statusCode="404" redirect="/Http404"/>
      <error statusCode="403" redirect="/Http403"/>
      <error statusCode="550" redirect="/Http550"/>
    </customErrors>
    <compilation debug="true" targetFramework="4.0"/>
    <pages controlRenderingCompatibilityVersion="4.0">
      <namespaces>
        <add namespace="System.Web.Helpers"/>
        <add namespace="System.Web.Mvc"/>
        <add namespace="System.Web.Mvc.Ajax"/>
        <add namespace="System.Web.Mvc.Html"/>
        <add namespace="System.Web.Routing"/>
        <add namespace="System.Web.WebPages"/>
      </namespaces>
    </pages>
  </system.web>
  <system.web.extensions>
    <scripting>
      <webServices>
        <jsonSerialization maxJsonLength="50000000"/>
      </webServices>
    </scripting>
    </system.web.extensions>
 <system.webServer>
 ....
</system.webServer>
</configuration>

Answer 1

According to the link you provided (the 2nd most voted answer), your web.config setting will be ignored because you're using an internal instance of the JavaScriptSerializer.

If you need the value to be stored in the web.config, you could add a key in the <appSettings> section called maxJsonLength with a value of 50000000 and then in your code, you could use it like:

var serializer = new JavaScriptSerializer();
serializer.MaxJsonLength = ConfigurationManager.AppSettings['maxJsonLength'];