namespace BlogEngine.Core

{

    using System;

    using System.IO;

    using System.Net;

    using System.Xml;



    /// <summary>

    /// Derived XmlUrlResolver class designed to prevent security problems with

    ///     dangerous XML input, by limiting the amount of data that can be retrieved.

    /// </summary>

    public class XmlSafeResolver : XmlUrlResolver

    {

        #region Constants and Fields



        /// <summary>

        /// The buffer size. 1 KB.

        /// </summary>

        private const int BufferSize = 1024;



        /// <summary>

        /// The max response size. 1 MB.

        /// </summary>

        private const int MaxResponseSize = 1024 * 1024;



        /// <summary>

        /// The timeout. 10 seconds.

        /// </summary>

        private const int Timeout = 10000;



        #endregion



        #region Public Methods



        /// <summary>

        /// Maps a URI to an object containing the actual resource.

        /// </summary>

        /// <param name="absoluteUri">

        /// The URI returned from <see cref="M:System.Xml.XmlResolver.ResolveUri(System.Uri,System.String)"/>

        /// </param>

        /// <param name="role">

        /// The current implementation does not use this parameter when resolving URIs. This is provided for future extensibility purposes. For example, this can be mapped to the xlink:role and used as an implementation specific argument in other scenarios.

        /// </param>

        /// <param name="typeOfObjectToReturn">

        /// The type of object to return. The current implementation only returns System.IO.Stream objects.

        /// </param>

        /// <returns>

        /// A System.IO.Stream object or null if a type other than stream is specified.

        /// </returns>

        /// <exception cref="T:System.Xml.XmlException">

        /// <paramref name="typeOfObjectToReturn"/> is neither null nor a Stream type.

        /// </exception>

        /// <exception cref="T:System.UriFormatException">

        /// The specified URI is not an absolute URI.

        /// </exception>

        /// <exception cref="T:System.ArgumentNullException">

        /// <paramref name="absoluteUri"/> is null.

        /// </exception>

        /// <exception cref="T:System.Exception">

        /// There is a runtime error (for example, an interrupted server connection).

        /// </exception>

        public override object GetEntity(Uri absoluteUri, string role, Type typeOfObjectToReturn)

        {

            if (absoluteUri.IsLoopback)

            {

                return null;

            }



            var request = WebRequest.Create(absoluteUri);

            request.Timeout = Timeout;



            var response = request.GetResponse();

            if (response == null)

            {

                throw new XmlException("Could not resolve external entity");

            }



            var responseStream = response.GetResponseStream();

            if (responseStream == null)

            {

                throw new XmlException("Could not resolve external entity");

            }



            responseStream.ReadTimeout = Timeout;



            var copyStream = new MemoryStream();

            var buffer = new byte[BufferSize];

            int bytesRead;

            var totalBytesRead = 0;

            do

            {

                bytesRead = responseStream.Read(buffer, 0, buffer.Length);

                totalBytesRead += bytesRead;

                if (totalBytesRead > MaxResponseSize)

                {

                    throw new XmlException("Could not resolve external entity");

                }



                copyStream.Write(buffer, 0, bytesRead);

            }

            while (bytesRead > 0);



            copyStream.Seek(0, SeekOrigin.Begin);

            return copyStream;

        }



        #endregion

    }

}