反序列化出错，不知如何解决

[eaglet]

[Serializable]
class copy
{
public int a ;
}

copy copyobj1 = new copy ();

IFormatter formatter = new BinaryFormatter ();
Stream stream = new MemoryStream();
formatter.Serialize(stream, copyobj1);
long len = stream.Length ;
copy copyobj2 =(copy)formatter.Deserialize(stream);

stream.Close();


执行到 copy copyobj2 =(copy)formatter.Deserialize(stream); 时出错
错误提示
An unhandled exception of type 'System.Runtime.Serialization.SerializationException' occurred in mscorlib.dll

Additional information: 在分析完成之前就遇到流结尾。
不知时什么原因

[brightheroes]

stream.Seek(0,SeekOrigin.Begin);//加上这个
copy copyobj2 =(copy)formatter.Deserialize(stream);

[eaglet]

高