[.net] Hexadecimal value 0x00 is a invalid character

I am generating an XML document from a StringBuilder, basically something like:

string.Format("<text><row>{0}</row><col>{1}</col><textHeight>{2}</textHeight><textWidth>{3}</textWidth><data>{4}</data><rotation>{5}</rotation></text>

Later, something like:

XmlDocument document = new XmlDocument();
document.LoadXml(xml);
XmlNodeList labelSetNodes = document.GetElementsByTagName("labels");
for (int index = 0; index < labelSetNodes.Count; index++)
{
    //do something
}

All the data comes from a database. Recently I've had a few issues with the error:

Hexadecimal value 0x00 is a invalid character, line 1, position nnnnn

But its not consistent. Sometimes some 'blank' data will work. The 'faulty' data works on some PCs, but not others.

In the database, the data is always a blank string. It is never 'null' and in the XML file, it comes out as < data>< /data>, i.e. no character between opening and closing. (but not sure if this can be relied on as I am pulling it from the 'immediate' window is vis studio and pasting it into textpad).

There is possibly differences in the versions of sql server (2008 is where it would fail, 2005 would work) and collation too. Not sure if any of these are likely causes?

But exactly the same code and data will sometimes fail. Any ideas where the problem lies?

I also get the same error in an ASP.NET application when I saved some unicode data (Hindi) in the Web.config file and saved it with "Unicode" encoding.

It fixed the error for me when I saved the Web.config file with "UTF-8" encoding.

To add to Sonz's answer above, following worked for us.

//Instead of 
XmlString.Replace("�", "[0x00]");
// use this
XmlString.Replace("\x00", "[0x00]");

I'm using IronPython here (same as .NET API) and reading the file as UTF-8 in order to properly handle the BOM fixed the problem for me:

xmlFile = Path.Combine(directory_str, 'file.xml')
doc = XPathDocument(XmlTextReader(StreamReader(xmlFile.ToString(), Encoding.UTF8)))

It would work as well with the XmlDocument:

doc = XmlDocument()
doc.Load(XmlTextReader(StreamReader(xmlFile.ToString(), Encoding.UTF8)))

As kind of a late answer:

I've had this problem with SSRS ReportService2005.asmx when uploading a report.

    Public Shared Sub CreateReport(ByVal strFileNameAndPath As String, ByVal strReportName As String, ByVal strReportingPath As String, Optional ByVal bOverwrite As Boolean = True)
        Dim rs As SSRS_2005_Administration_WithFOA = New SSRS_2005_Administration_WithFOA
        rs.Credentials = ReportingServiceInterface.GetMyCredentials(strCredentialsURL)
        rs.Timeout = ReportingServiceInterface.iTimeout
        rs.Url = ReportingServiceInterface.strReportingServiceURL
        rs.UnsafeAuthenticatedConnectionSharing = True

        Dim btBuffer As Byte() = Nothing

        Dim rsWarnings As Warning() = Nothing
        Try
            Dim fstrStream As System.IO.FileStream = System.IO.File.OpenRead(strFileNameAndPath)
            btBuffer = New Byte(fstrStream.Length - 1) {}
            fstrStream.Read(btBuffer, 0, CInt(fstrStream.Length))
            fstrStream.Close()
        Catch ex As System.IO.IOException
            Throw New Exception(ex.Message)
        End Try

        Try
            rsWarnings = rs.CreateReport(strReportName, strReportingPath, bOverwrite, btBuffer, Nothing)

            If Not (rsWarnings Is Nothing) Then
                Dim warning As Warning
                For Each warning In rsWarnings
                    Log(warning.Message)
                Next warning
            Else
                Log("Report: {0} created successfully with no warnings", strReportName)
            End If

        Catch ex As System.Web.Services.Protocols.SoapException
            Log(ex.Detail.InnerXml.ToString())
        Catch ex As Exception
            Log("Error at creating report. Invalid server name/timeout?" + vbCrLf + vbCrLf + "Error Description: " + vbCrLf + ex.Message)
            Console.ReadKey()
            System.Environment.Exit(1)
        End Try
    End Sub ' End Function CreateThisReport

The problem occurs when you allocate a byte array that is at least 1 byte larger than the RDL (XML) file.

Specifically, I used a C# to vb.net converter, that converted

  btBuffer = new byte[fstrStream.Length];

into

  btBuffer = New Byte(fstrStream.Length) {}

But because in C# the number denotes the NUMBER OF ELEMENTS in the array, and in VB.NET, that number denotes the UPPER BOUND of the array, I had an excess byte, causing this error.

So the problem's solution is simply:

  btBuffer = New Byte(fstrStream.Length - 1) {}

In my case, it took some digging, but found it.

My Context

I'm looking at exception/error logs from the website using Elmah. Elmah returns the state of the server at the of time the exception, in the form of a large XML document. For our reporting engine I pretty-print the XML with XmlWriter.

During a website attack, I noticed that some xmls weren't parsing and was receiving this '.', hexadecimal value 0x00, is an invalid character. exception.

NON-RESOLUTION: I converted the document to a byte[] and sanitized it of 0x00, but it found none.

When I scanned the xml document, I found the following:

...
<form>
...
<item name="SomeField">
   <value
     string="C:\boot.ini&#x0;.htm" />
 </item>
...

There was the nul byte encoded as an html entity � !!!

RESOLUTION: To fix the encoding, I replaced the � value before loading it into my XmlDocument, because loading it will create the nul byte and it will be difficult to sanitize it from the object. Here's my entire process:

XmlDocument xml = new XmlDocument();
details.Xml = details.Xml.Replace("�", "[0x00]");  // in my case I want to see it, otherwise just replace with ""
xml.LoadXml(details.Xml);

string formattedXml = null;

// I have this in a helper function, but for this example I have put it in-line
StringBuilder sb = new StringBuilder();
XmlWriterSettings settings = new XmlWriterSettings {
    OmitXmlDeclaration = true,
    Indent = true,
    IndentChars = "\t",
    NewLineHandling = NewLineHandling.None,
};
using (XmlWriter writer = XmlWriter.Create(sb, settings)) {
    xml.Save(writer);
    formattedXml = sb.ToString();
}

LESSON LEARNED: sanitize for illegal bytes using the associated html entity, if your incoming data is html encoded on entry.