Reading IIS request filtering maximal request length

IisMaxRequestLength.cs

using System;
using System.IO;
using System.Linq;
using System.Web;
using System.Web.Configuration;
using System.Xml;

namespace lennybacon.Samples
{
  class IisMaxRequestLength
  {
    private static int? GetMaxRequestLengthOfRequestFiltering()
    {
      const int requestFilteringDefaultLimit = 30000000;

      var configuration = WebConfigurationManager.OpenWebConfiguration("~");

      var webServerSection =
        configuration.GetSection("system.webServer");

      var xml = webServerSection.SectionInformation.GetRawXml();
      var dom = new XmlDocument();
      dom.LoadXml(xml);

      var requestLimits =
        dom.SelectSingleNode(
          "/system.webServer/security/requestFiltering/requestLimits");
      var requestFilteringInstalled = false;
      if (requestLimits == null)
      {
        var appPoolName =
          HttpContext.Current.Request.ServerVariables["APP_POOL_ID"];
        var windowsDrive =
          new DirectoryInfo(
            Environment.GetFolderPath(Environment.SpecialFolder.System)).Root;

        var appPoolTempCfgPath =
          Path.Combine(
            windowsDrive.FullName,
            "inetpub",
            "temp",
            "appPools",
            appPoolName,
            appPoolName + ".config");

        dom = new XmlDocument();
        dom.Load(appPoolTempCfgPath);

        requestFilteringInstalled =
          dom.SelectSingleNode(
            "//system.webServer/security/requestFiltering") == null;

        requestLimits =
          dom.SelectSingleNode(
            "//system.webServer/security/requestFiltering/requestLimits");
      }

      if (requestLimits != null)
      {
        const string maxAllowedContentLengthName = "maxAllowedContentLength";
        var attribute =
          requestLimits.Attributes?.Cast<XmlAttribute>()
            .FirstOrDefault(
              a =>
                maxAllowedContentLengthName.Equals(
                  a.Name,
                  StringComparison.Ordinal)
            );
        if (attribute != null)
        {
          var requestFilteringLimit = 0;
          if (int.TryParse(attribute.Value, out requestFilteringLimit))
          {
            return requestFilteringLimit;
          }
        }
      }
      if (!requestFilteringInstalled)
      {
        return null;
      }
      return requestFilteringDefaultLimit;
    }
  }
}