问XmlSerializer和XmlException -缺少根元素

EN

System.InvalidOperationException was unhandled
  Message=There is an error in XML document (0, 0).
  Source=System.Xml
  StackTrace:
   at System.Xml.Serialization.XmlSerializer.Deserialize(XmlReader xmlReader, String encodingStyle, XmlDeserializationEvents events)
   at System.Xml.Serialization.XmlSerializer.Deserialize(Stream stream)
   at TestSerilalization.Program.Main(String[] args) in D:\MyDocs\Projects\dsb\Src\Apps\RecipeMgr\Other\TestSerlialization\Program.cs:line 45
   at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
   at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
   at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
   at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ThreadHelper.ThreadStart()
 InnerException: System.Xml.XmlException
   Message=Root element is missing.
   Source=System.Xml
   LineNumber=0
   LinePosition=0
   SourceUri=""
   StackTrace:
        at System.Xml.XmlTextReaderImpl.Throw(Exception e)
        at System.Xml.XmlTextReaderImpl.ParseDocumentContent()
        at System.Xml.XmlTextReaderImpl.Read()
        at System.Xml.XmlTextReader.Read()
        at System.Xml.XmlReader.MoveToContent()
        at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationReaderClass1.Read4_Class1()
   InnerException: 

static void Main(string[] args)
{
XmlSerializer xmlSerializer = new System.Xml.Serialization.XmlSerializer( typeof( Class1 ), "http://xxx" );

Class1 c1 = new Class1();

FileStream strm = File.Create( "d:\\temp\\classes.xml" );
xmlSerializer.Serialize( strm, c1 );
strm.Close();

Class1 c2 = null;
FileStream strm2 = File.Create( "d:\\temp\\classes.xml" );
XmlSerializer xmlSerializer2 = new System.Xml.Serialization.XmlSerializer( typeof( Class1 ), "http://xxx" );
c2 = (Class1)xmlSerializer2.Deserialize( strm2 );
strm2.Close();
}

public class Class1
{
public Class1()
  {
  m_Class2 = new Class2();
  }

public Class2 class2
  {
  get { return m_Class2; }
  set { m_Class2 = value; }
  }

private Class2  m_Class2;
}

public class Class2
{
public Class2()
  {
  m_Int = 999;
  m_Str = "sdfsdfsdsd";
  }

public int getInt
  {
  get { return m_Int; }
  set { m_Int = value; }
  }

public string getStr
  {
  get { return m_Str; }
  set { m_Str = value; }
  }

private int     m_Int;
private string  m_Str;
}

<?xml version="1.0"?>
<Class1 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns="http://xxx">
  <class2>
    <getInt>999</getInt>
    <getStr>sdfsdfsdsd</getStr>
  </class2>
</Class1>