How do I differentiate types of XML files before deserializing?

Here's a way to do it by using an XDocument to parse the file, read the root element to determine the type, and read it into your serializer.

var xdoc = XDocument.Load(filePath);Type type;if (xdoc.Root.Name.LocalName == "score-partwise")    type = typeof(ScorePartwise);else if (xdoc.Root.Name.LocalName == "score-timewise")    type = typeof(ScoreTimewise);else    throw new Exception();var xmlSerializer = new XmlSerializer(type);var result = xmlSerializer.Deserialize(xdoc.CreateReader());

I would create both serializers

var partwiseSerializer = new XmlSerializer(typeof(ScorePartwise));var timewiseSerializer = new XmlSerializer(typeof(ScoreTimewise));

Assuming that there is only these two I would call CanDeserialize method on one

using (var fileStream = new FileStream(openFileDialog.FileName, FileMode.Open)){  using (var xmlReader = XmlReader.Create(filStream))  {    if (partwiseSerializer.CanDeserialize(xmlReader))    {       var result = partwiseSerializer.Deserialize(xmlReader);    }    else    {       var result = timewiseSerializer.Deserialize(xmlReader);    }  }}

Obviously this is just an idea how to do it. If there were more options or according to your application design I would use a more sophisticated way to call CanDeserialize, but that method is the key in my opinion:

http://msdn.microsoft.com/en-us/library/system.xml.serialization.xmlserializer.candeserialize.aspx

The XmlReader class can be found here:

http://msdn.microsoft.com/en-us/library/System.Xml.XmlReader(v=vs.110).aspx

If you're concerned about resource usage:

    internal const string NodeStart = "<Error ";    public static bool IsErrorDocument(string xml)    {        int headerLen = 1;        if (xml.StartsWith(Constants.XMLHEADER_UTF8))        {            headerLen += Constants.XMLHEADER_UTF8.Length;        }        else if (xml.StartsWith(Constants.XMLHEADER_UTF16))        {            headerLen += Constants.XMLHEADER_UTF16.Length;        }        else        {            return false;        }        if (xml.Length < headerLen + NodeStart.Length)        {            return false;        }        return xml.Substring(headerLen, NodeStart.Length) == NodeStart;    }internal class Constants{    public const string XMLHEADER_UTF16 = "<?xml version=\"1.0\" encoding=\"utf-16\"?>";    public const string XMLHEADER_UTF8 = "<?xml version=\"1.0\" encoding=\"utf-8\"?>";}