Nested non-nullable elements in choice not supported

[jmatss]

Because of how XmlSchemaClassGenerator generates C# properties for choices (see choice), all properties generated from items in a choice must be nullable.

The current implementation of XmlSchemaClassGenerator only forces the direct children of choices to nullable. It does not force any descendants nested deeper than its children to nullable and might therefore generate invalid code in those cases.

For example the following schema file:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="Root">
    <xs:complexType>
      <xs:choice>
        <xs:sequence>
          <xs:element name="Element" type="xs:int"/>
        </xs:sequence>
        <xs:group ref="GroupRef"/>
      </xs:choice>
    </xs:complexType>
  </xs:element>

  <xs:group name="GroupRef">
    <xs:sequence>
      <xs:element name="GroupRefChild" type="xs:int"/>
    </xs:sequence>
  </xs:group>
</xs:schema>

generates a class that looks something like this:

public partial class Root
{
    [System.ComponentModel.DataAnnotations.RequiredAttribute()]
    public int Element { get; set; }

    [System.ComponentModel.DataAnnotations.RequiredAttribute()]
    public int GroupRefChild { get; set; }
}

which isn't valid since both the properties are required when they must be nullable according to how XmlSchemaClassGenerator handles choice elements.