If in a class you have a value type property that is optional, it's common to specify it as nullable. However, if you serialize a nullable value type property to XML, you'll get something like this if it's null.

<Value xsi:nil="true" />

Or, if you turned off declaration of the XML schema instance namespace, it will be even more verbose.

<Value p2:nil="true" xmlns:p2="http://www.w3.org/2001/XMLSchema-instance" />

This might look strange but it will properly deserialize. Nonetheless, you might have a reason to have a nullable value type property serialize the way a class property does, which is to not appear in the XML output at all if it's null. Setting the IsNullable property of the XmlElement attribute for such a property will have no effect. (Well, setting it to false will result in a run-time error.)

The XmlElementAttribute.IsNullable Property page at MSDN has this to say.

You cannot apply the IsNullable property to a member typed as a value type because a value type cannot contain null. Additionally, you cannot set this property to false for nullable value types. When such types are null, they will be serialized by setting xsi:nil to true.

One work-around is to declare another property that is a class type, such as string, and parse it in the nullable value property.

[XmlElement("Value")]
public string ValueString { get; set; }

[XmlIgnore]
public int? Value
{
    get { return int.Parse(ValueString); }
    set { ValueString= value.ToString(); }
}

Of course, you can go in the opposite direction as well.

[XmlElement("Value")]
public string ValueString
{
    get { return Value.HasValue ? Value.Value.ToString() : null; }
    set
    {
        if(value != null)
            Value= int.Parse(value);
        else
            Value= null;
    }
}

[XmlIgnore]
public int? Value { get; set; }