反序列化对象时的ArgumentNullException

Question

我可以在soap格式中序列化我的对象，我在字符串变量fullPath中编写了目录但是当我尝试反序列化它时，它会显示ArgumentNullException，为什么？

//The code in form1.
Train tr = new Train();
fullPath = "D:\\file.xml";
tr.WriteToFile(fullPath);

//WriteToFile() method in Train class.
public void WriteToFile(string path)
{
    using (FileStream stream = new FileStream(path, FileMode.Create))
    {
        SoapFormatter formatter = new SoapFormatter();
        formatter.Serialize(stream, this);
     }
}

//The code in Form2
Form1 f = new Form1();
string fullPath = f.fullPath;
try
{
     using (Stream stream = File.OpenRead(f.fullPath))
     {
          SoapFormatter formatter = new SoapFormatter();
          tr = formatter.Deserialize(stream) as Train;
     }
 }
 catch (ArgumentNullException ex)
 {
      MessageBox.Show("ArgumentNullException: " + ex.Message);
 }