序列化的问题，请大侠帮看看问题在哪里？

Question

Flow flow = new Flow(FlowName,FlowType,FlowFormAddress);
for(int i = 0; i < this.lbxStep.Items.Count; i ++)
{
flow.AddStep((Step)lbxStep.Items[i]);
} try
{
System.Xml.Serialization.XmlSerializer ser = new System.Xml.Serialization.XmlSerializer(typeof(Flow));
if(System.IO.File.Exists(@"C:\flow.xml"))
{
System.IO.TextWriter writer = new System.IO.StreamWriter(@"C:\flow.xml");
ser.Serialize(writer,flow);
writer.Close();
}
}
执行ser.Serialize(writer,flow);这一句时就会出错，错误信息是：生成XML文档时出错。实在是看不出来哪里有错误啊，请大家帮忙了！

Answer 1

Flow 必须实现ISerializable 接口