serialization of List error

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C#

given the following code 
[Serializable]
    public class check
    {
        public check()
        {
            //int_array = new List<int>();
            string_array = new List<string>();
        }

        [XmlArray("string_array",Order=1)]
        [XmlArrayItem("string_element",typeof(string))]
        public List<string> string_array;

        //[XmlArray("int_array")]
        //[XmlArrayItem("int_element")]
        //public List<int> int_array;
        public void write_to_xml(string path)
        {
            XmlSerializer serailizer = new XmlSerializer(typeof(check));
            TextWriter writer = new StreamWriter(path);
            serailizer.Serialize(writer, this);
            writer.Close();
        }
        static void Main()
        {
           //check class
            check c = new check();
            c.string_array.AddRange(new string[] { "string1", "string2", "string3" });
            c.write_to_xml(@"E:\check.xml");
        }
    }

this code run OK and XML file generated is

<?xml version="1.0" encoding="utf-8"?>
<check xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <string_array>
    <string_element>string1</string_element>
    <string_element>string2</string_element>
    <string_element>string3</string_element>
  </string_array>
</check>

now remove comments in previous class and hit F5 again
what happen
this error appear There was an error reflecting type 'FET.check'.
Why this error appear?
IS serialized class must contain only one instance of XmlArray attribute ? or what?