使用我们的应用程序打开SVG时DTD上的错误403（禁止）

Question

在我们的C#应用程序中，有时应用程序需要打开一个svg文件。 为此，使用以下代码：

XmlReaderSettings settings = new XmlReaderSettings();
settings.ProhibitDtd = false;
settings.ValidationType = ValidationType.None;
XmlReader xr = XmlReader.Create(serverMainPath + @"/path/to/svg/file.svg", settings);
XmlDocument xd = new XmlDocument();
_nmsm = new XmlNamespaceManager(xd.NameTable);
_nmsm.AddNamespace("svg", "http://www.w3.org/2000/svg");
_nmsm.AddNamespace("v", "http://schemas.microsoft.com/visio/2003/SVGExtensions/");
xd.Load(xr);

我们使用的标准svg开头（据我所知它是一个有效的svg）：

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
    <!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.0//EN" "http://www.w3.org/TR/2001/REC-SVG-20010904/DTD/svg10.dtd">
    <!-- Généré par Microsoft Visio 11.0, SVG Export, v1.0 svgIncluded.svg Page-1 -->
    <svg xmlns="http://www.w3.org/2000/svg" xmlns:v="http://schemas.microsoft.com/visio/2003/SVGExtensions/" width="8.26772in" height="11.6929in" viewBox="0 0 595.276 841.889" xml:space="preserve" color-interpolation-filters="sRGB" class="st23" >

在过去的6个月里它运作得很好。 但是，由于一到两周，此代码有时会产生以下错误：

Server Error in '/ourApp' Application.
--------------------------------------------------------------------------------

The remote server returned an error: (403) Forbidden. 
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.Net.WebException: The remote server returned an error: (403) Forbidden.

Source Error: 

Line 40:                 _nmsm.AddNamespace("svg", "http://www.w3.org/2000/svg");
Line 41:                 _nmsm.AddNamespace("v", "http://schemas.microsoft.com/visio/2003/SVGExtensions/");
Line 42:                 xd.Load(xr);


Source File: path/to/the/file/file.cs    Line: 42 

Stack Trace: 

[WebException: The remote server returned an error: (403) Forbidden.]
   System.Net.HttpWebRequest.GetResponse() +5400333
   System.Xml.XmlDownloadManager.GetNonFileStream(Uri uri, ICredentials credentials) +69
   System.Xml.XmlDownloadManager.GetStream(Uri uri, ICredentials credentials) +3929515
   System.Xml.XmlUrlResolver.GetEntity(Uri absoluteUri, String role, Type ofObjectToReturn) +54
   System.Xml.XmlTextReaderImpl.OpenStream(Uri uri) +34
   System.Xml.XmlTextReaderImpl.DtdParserProxy_PushExternalSubset(String systemId, String publicId) +380

[XmlException: An error has occurred while opening external DTD 'http://www.w3.org/TR/2001/REC-SVG-20010904/DTD/svg10.dtd': The remote server returned an error: (403) Forbidden.]
   System.Xml.XmlTextReaderImpl.Throw(Exception e) +76
   System.Xml.XmlTextReaderImpl.DtdParserProxy_PushExternalSubset(String systemId, String publicId) +513
   System.Xml.DtdParserProxy.System.Xml.IDtdParserAdapter.PushExternalSubset(String systemId, String publicId) +16
   System.Xml.DtdParser.ParseExternalSubset() +21
   System.Xml.DtdParser.ParseInDocumentDtd(Boolean saveInternalSubset) +4017069
   System.Xml.DtdParser.Parse(Boolean saveInternalSubset) +54
   System.Xml.DtdParserProxy.Parse(Boolean saveInternalSubset) +31
   System.Xml.XmlTextReaderImpl.ParseDoctypeDecl() +254
   System.Xml.XmlTextReaderImpl.ParseDocumentContent() +451
   System.Xml.XmlTextReaderImpl.Read() +151
   System.Xml.XmlLoader.LoadNode(Boolean skipOverWhitespace) +58
   System.Xml.XmlLoader.LoadDocSequence(XmlDocument parentDoc) +20
   System.Xml.XmlLoader.Load(XmlDocument doc, XmlReader reader, Boolean preserveWhitespace) +129
   System.Xml.XmlDocument.Load(XmlReader reader) +108
   OurMethod(params) in path/to/the/file/file.cs:42
   StackTrace in our App.

--------------------------------------------------------------------------------
Version Information: Microsoft .NET Framework Version:2.0.50727.3643; ASP.NET Version:2.0.50727.3634 

起初我认为这是由于我们的互联网代理政策发生了变化，但由于它已经被发现在另一个不同代理的应用程序部署中，我非常怀疑。

错误似乎或多或少随机，有时它会起作用，有时它不会。 我只是不知道为什么会这样做。

问题是：你知道为什么会发生这种情况吗？ 如果是，你知道如何解决它吗？ 如果不是，任何解决方法的想法？

Answer 1

最后，我用了一个自制的班级来完成这项工作。 它的灵感来自XmlUrlResolver扩展的MSDN实现 。 感谢@mzjn指出了指向MSDN资源的链接。

using System;
using System.Net;
using System.Net.Cache;
using System.Xml;
using System.IO;

namespace My.Nasmespace.Class.IO
{
    class XmlExtendedResolver : XmlUrlResolver
    {
        bool enableHttpCaching;
        ICredentials credentials;
        private string localServerPath;
        //resolve ressource from localServerPath if it's possible
        //resolve resources from cache (if possible) when enableHttpCaching is set to true 
        //resolve resources from source when enableHttpcaching is set to false  
        public XmlExtendedResolver(bool enableHttpCaching, string serverPath)
        {
            this.enableHttpCaching = enableHttpCaching;
            localServerPath = serverPath;
        }

        public override ICredentials Credentials
        {
            set
            {
                credentials = value;
                base.Credentials = value;
            }
        }

        public override object GetEntity(Uri absoluteUri, string role, Type ofObjectToReturn)
        {
            if (absoluteUri == null)
            {
                throw new ArgumentNullException("absoluteUri");
            }
            //resolve resources from cache (if possible) 
            if (absoluteUri.Scheme == "http" && enableHttpCaching && (ofObjectToReturn == null || ofObjectToReturn == typeof(Stream)))
            {
                /*Try to resolve it from the local path if it exists*/
                if (!string.IsNullOrEmpty(localServerPath) && absoluteUri.AbsolutePath.EndsWith(".dtd") && !absoluteUri.AbsoluteUri.StartsWith(localServerPath))
                {
                    try { 
                        return GetEntity(new Uri(localServerPath + "/Xml/" + absoluteUri.Segments[absoluteUri.Segments.Length-1]), role, ofObjectToReturn);
                    } catch (FileNotFoundException){
                         //Fail silently to go to the web request.
                    }
                }
                WebRequest webReq = WebRequest.Create(absoluteUri);
                webReq.CachePolicy = new    HttpRequestCachePolicy(HttpRequestCacheLevel.Default);
                if (credentials != null)
                {
                    webReq.Credentials = credentials;
                }
                WebResponse resp = webReq.GetResponse();
                return resp.GetResponseStream();
            }
            //otherwise use the default behavior of the XmlUrlResolver class (resolve resources from source) 
        else
            {
                return base.GetEntity(absoluteUri, role, ofObjectToReturn);
            }
        }
    }
}

Answer 2

我认为你必须尝试一些逻辑，我只是​​从一些地方缓冲

request.UserAgent = "userAgents";
request.Accept = "*/*";

和

request.Credentials = new NetworkCredentials("username", "password");

or 

 request.Credentials = CredentialCache.DefaultCredentials;