Deserialize empty XML element as Guid.Empty

Question

I have some troubles with deserializing.

<Order>
    ...
    <CardNumber />
    ...
</Order>

If I use

<CardNumber>00000000-0000-0000-0000-000000000000</CardNumber>

it's working normally, but in case when I use just <CardNumber /> - object is not deserializing (

Is there any way to deserialize empty element as Guid.Empty?

Property which should be mapped with value of this element:

[XmlElement(ElementName = "CardNumber")]
[JsonProperty("CardNumber")]
public Guid? CardNumber { get; set; }

Same situation in JSON works normally and use Guid.Empty instead of empty element value

{
    "CardNumber": ""
}

Answer 1

The exception you are seeing explains the problem clearly:

System.InvalidOperationException occurred
  Message="There is an error in XML document (3, 3)."
  InnerException: System.FormatException
       Message="Guid should contain 32 digits with 4 dashes (xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx)."

As stated, XmlSerializer does not support deserializing an empty string to a Guid. Thus you will need to do the conversion manually using a surrogate property:

[XmlRoot("Order")]
public class Order
{
    [XmlIgnore]
    [JsonProperty("CardNumber")]
    public Guid? CardNumber { get; set; }

    [XmlElement(ElementName = "CardNumber", IsNullable = true)]
    [Browsable(false), EditorBrowsable(EditorBrowsableState.Never), DebuggerBrowsable(DebuggerBrowsableState.Never)]
    [JsonIgnore]
    public string XmlCardNumber
    {
        get
        {
            if (CardNumber == null)
                return null;
            else if (CardNumber.Value == Guid.Empty)
                return "";
            return XmlConvert.ToString(CardNumber.Value);
        }
        set
        {
            if (value == null)
                CardNumber = null;
            else if (string.IsNullOrEmpty(value))
                CardNumber = Guid.Empty;
            else
                CardNumber = XmlConvert.ToGuid(value);
        }
    }
}

If this is something you need to do in many different types that have Guid? properties, you can extract a surrogate type like so:

[XmlType(AnonymousType = true, IncludeInSchema = false)]
public class XmlGuid
{
    [XmlIgnore]
    public Guid Guid { get; set; }

    [XmlText]
    public string XmlCardNumber
    {
        get
        {
            if (Guid == Guid.Empty)
                return null;
            return XmlConvert.ToString(Guid);
        }
        set
        {
            if (string.IsNullOrEmpty(value))
                Guid = Guid.Empty;
            else
                Guid = XmlConvert.ToGuid(value);
        }
    }

    public static implicit operator Guid?(XmlGuid x)
    {
        if (x == null)
            return null;
        return x.Guid;

    }

    public static implicit operator XmlGuid(Guid? g)
    {
        if (g == null)
            return null;
        return new XmlGuid { Guid = g.Value };
    }

    public static implicit operator Guid(XmlGuid x)
    {
        if (x == null)
            return Guid.Empty;
        return x.Guid;

    }

    public static implicit operator XmlGuid(Guid g)
    {
        return new XmlGuid { Guid = g };
    }
}

And use it like:

[XmlRoot("Order")]
public class Order
{
    [XmlElement(Type = typeof(XmlGuid), ElementName = "CardNumber", IsNullable = true)]
    [JsonProperty("CardNumber")]
    public Guid? CardNumber { get; set; }
}

Here I am taking advantage of the fact that the XmlElementAttribute.Type property automatically picks up the implicit conversion I defined for Guid? from and to XmlGuid.

Answer 2

Null is not the same as Guid.Empty. In the JSON serializer, you denote null using an empty string.

If you serialize your class using XmlSerializer you'll see it uses xsi:nil="true" to denote a null value.

For example:

<Order xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
       xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <CardNumber xsi:nil="true" />
</Order>