C# xml serialization required field

Question

I need to mark some fields as required to be writen in XML file, but no success. I have a configuration class with ~30 properties, this is why i cannot envelope all properties like this

    public string SomeProp
    {
        get { return _someProp; }
        set
        {
            if (_someProp == null)
                throw  new ArgumentNullException("value");
            _someProp = value;
        }
    }

and null is valid value from code, but not from XML. I'm trying to use XmlAttribute, but it doesn't affect like I expected (IOException when deserialising).

For example this code:

    [XmlElement(IsNullable = true)]
    public String Name { get; set; }

    [XmlElement(IsNullable = true)]
    public String Description { get; set; }

is pretty valid when Name and Description tags are missing.

---

clarification:

Class:

public class MyClass
{
    [XmlElement(IsNullable = true)]
    public String Name { get; set; }

    [XmlElement(IsNullable = true)]
    public String Description { get; set; } 
}

XML:

<?xml version="1.0"?>
<MyClass xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <Name>hello</Name>
</MyClass>

code:

MyClass deserialize = (MyClass)new XmlSerializer(my.GetType()).Deserialize(new FileStream("result.xml", FileMode.Open));
Console.WriteLine(deserialize.Name);
Console.WriteLine(deserialize.Description);

result: deserialization is successful, Description is null

expected: exception, Description tag not found.

Answer 1

I believe you could follow two approaches. One would be the use of a validating XmlReader that uses a schema, the other one is a reflection check AFTER the deserialization. The last option is the solution I provide here:

This helper takes a stream and deserialize that given the generic type to an instance of that type. Once that object exists its properties can be iterated and checked if the property is decorated with the XmlElementAttribute. If that attribute is found its IsNullable property is checked and if false AND the property on the object instance is false we throw an exception.

public T ValidatingDeserializer<T>(Stream s)
{
    T deserialize = (T)new XmlSerializer( typeof(T)).Deserialize(s);

    foreach(var pi in typeof(T).GetProperties())
    {
        var xmlnull = pi.GetCustomAttribute(typeof(XmlElementAttribute));
        if (xmlnull!=null)
        {   var xmlnullInst = (XmlElementAttribute) xmlnull;
            if (!xmlnullInst.IsNullable && pi.GetValue(deserialize)==null)
            {
                throw new Exception(String.Format("{0} is null", pi.Name));
            }
        }
    }
    return deserialize;
}

Usage

MyClass deserialize = ValidatingDeserializer<MyClass>(
     new FileStream("result.xml", FileMode.Open));

Console.WriteLine(deserialize.Name);
Console.WriteLine(deserialize.Description);

Alternative with a validating based on a schema

By using a schema you could verify if the supplied XML document is valid against the given schema.

ReaderFactory

This takes an xml stream and a schema stream, hooksup an event where errors are raised and returns an XmlReader that will be the input for the Deserialize method.

public XmlReader ReaderFactory(Stream xml, Stream schema)
{
    XmlSchemaSet sc = new XmlSchemaSet();

    // Add the schema to the collection.
    sc.Add(null, XmlReader.Create(schema));

    // Set the validation settings.
    XmlReaderSettings settings = new XmlReaderSettings();
    settings.ValidationType = ValidationType.Schema;
    settings.ValidationFlags |= XmlSchemaValidationFlags.ReportValidationWarnings;

    settings.Schemas = sc;
    settings.ValidationEventHandler += new ValidationEventHandler (ValidationCallBack);

    return XmlReader.Create(xml, settings);
}

The ValidateEvent target

 private static void ValidationCallBack(object sender, ValidationEventArgs e) {
    Console.WriteLine(
        "Validation Error: {0} near line: {1}, pos: {2}", 
        e.Message, 
        e.Exception.LineNumber, 
        e.Exception.LinePosition);
    throw new Exception("illegal xml content");
  }

Used sample schema for the xml shown in your question

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            xmlns=""
            elementFormDefault="qualified">

    <xsd:element name="MyClass" >
        <xsd:complexType>
            <xsd:sequence maxOccurs="1">
                <xsd:element name="Name" type="xsd:string"/>
                <xsd:element name="Description" type="xsd:string" minOccurs="1">
                </xsd:element>
            </xsd:sequence>
        </xsd:complexType>
    </xsd:element>

</xsd:schema>

Answer 2

Do you have to use XmlSerializer? DataContractSerializer provides the following option:

[DataMember(IsRequired = true)]