XML Serialization of an Object Containing invalid chars

I set the XmlReaderSettings property CheckCharacters to false. I would only advise doing this if you have serialized the data yourself via XmlSerializer. If it's from an unknown source then it's not really a good idea.

public static T Deserialize<T>(string xml)
{
    var xmlReaderSettings = new XmlReaderSettings() { CheckCharacters = false };

    XmlReader xmlReader = XmlTextReader.Create(new StringReader(xml), xmlReaderSettings);
    XmlSerializer xs = new XmlSerializer(typeof(T));

    return (T)xs.Deserialize(xmlReader);
}

You should really post the code of the class you're trying to serialize and deserialize. In the meantime, I'll make a guess.

Most likely, the invalid character is in a field or property of type string. You will need to serialize that as an array of bytes, assuming you can't avoid having that character present at all:

[XmlRoot("root")]
public class HasBase64Content
{
    internal HasBase64Content()
    {
    }

    [XmlIgnore]
    public string Content { get; set; }

    [XmlElement]
    public byte[] Base64Content
    {
        get
        {
            return System.Text.Encoding.UTF8.GetBytes(Content);
        }
        set
        {
            if (value == null)
            {
                Content = null;
                return;
            }

            Content = System.Text.Encoding.UTF8.GetString(value);
        }
    }
}

This produces XML like the following:

<?xml version="1.0" encoding="utf-8"?>
<root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xmlns:xsd="http://www.w3.org/2001/XMLSchema">
    <Base64Content>AAECAwQFFA==</Base64Content>
</root>

I see you'd probably prefer VB.NET:

''# Prettify doesn't like attributes as the first item in a VB code block, so this comment is here so that it looks right on StackOverflow.
<XmlRoot("root")> _
Public Class HasBase64Content

    Private _content As String
    <XmlIgnore()> _
    Public Property Content() As String
        Get
            Return _content
        End Get
        Set(ByVal value As String)
            _content = value
        End Set
    End Property

    <XmlElement()> _
    Public Property Base64Content() As Byte()
        Get
            Return System.Text.Encoding.UTF8.GetBytes(Content)
        End Get
        Set(ByVal value As Byte())
            If Value Is Nothing Then
                Content = Nothing
                Return
            End If
            Content = System.Text.Encoding.UTF8.GetString(Value)
        End Set
    End Property
End Class

I would exepct .NET to handle this, but you can also have look at XmlSerializer class and XmlReaderSettings (see sample generic method below):

public static T Deserialize<T>(string xml)
{
    var xmlReaderSettings = new XmlReaderSettings()
                                {
                                    ConformanceLevel = ConformanceLevel.Fragment,
                                    ValidationType = ValidationType.None
                                };

    XmlReader xmlReader = XmlTextReader.Create(new StringReader(xml), xmlReaderSettings);
    XmlSerializer xs = new XmlSerializer(typeof(T), "");

    return (T)xs.Deserialize(xmlReader);
}

I would also check if there are no encoding (Unicode, UTF8, etc.) issues in your code. Hexadecimal value 0x14 is not a char you would expect in XML :)