`System.Xml.XmlSerializer` cannot serialize `System.Nullable<t>` as attribute

[shmuelie]

System.InvalidOperationException occurred
  HResult=-2146233079
  Message=Cannot serialize member 'ResX' of type System.Nullable`1[System.Double]. XmlAttribute/XmlText cannot be used to encode complex types.
  Source=System.Xml.XmlSerializer
  StackTrace:
       at System.Xml.Serialization.XmlReflectionImporter.ImportAccessorMapping(MemberMapping accessor, FieldModel model, XmlAttributes a, String ns, Type choiceIdentifierType, Boolean rpc, Boolean openModel, RecursionLimiter limiter)
  InnerException: 

[joshfree]

cc: @zhenlan

[khdang]

@SamuelEnglard can you provide an example to reproduce this issue?

[shmuelie]

using System;

namespace SerializeNull
{
    public class Program
    {
        public class TestSerialize
        {
            [System.Xml.Serialization.XmlAttribute]
            public double? Attr
            {
                get;
                set;
            }
        }

        public static void Main(string[] args)
        {
            System.Xml.Serialization.XmlSerializer s = new System.Xml.Serialization.XmlSerializer(typeof(TestSerialize));
            using (System.IO.MemoryStream m = new System.IO.MemoryStream())
            {
                s.Serialize(m, new TestSerialize() { Attr = 1 });
                m.Seek(0, System.IO.SeekOrigin.Begin);
                Console.WriteLine((new System.IO.StreamReader(m)).ReadToEnd());
            }
            Console.ReadLine();
        }
    }
}

This should do it

[SGuyGe]

Does this also happen on desktop .NET?

[shmuelie]

@SGuyGe Can reproduce in Desktop .NET 2.0 through 4.6.2

[karelz]

Someone needs to debug it and find out which code to change.

[huanwu]

The fix is more complex than expected. Move to future. Refer dotnet/corefx#30536.