Serialization Custom DataType

Question

**I Convert XSD to c# Class now when i create object of class and assign a value am getting error

Cannot implicitly convert type 'int' to 'Frameworkgo.Envelope.IdentifierType

am doing serialization kindly help me on that

THANKYOU

static void Main(string[] args)
    {

        ObjectEnvelopeType objectenvelopetype = new ObjectEnvelopeType();
        objectenvelopetype.DocumentReference.DocumentID = 345;

        XmlSerializer serializer = new XmlSerializer(typeof(ObjectEnvelopeType));
        FileStream fs = new FileStream("Object.xml", FileMode.Create);
        serializer.Serialize(fs, objectenvelopetype);
        fs.Close();

        Console.ReadKey();
    }
public partial class ObjectEnvelopeType
{
 private ReferenceType documentReferenceField;
 public ReferenceType DocumentReference{get{return this.documentReferenceField;}set{documentReferenceField = value;}}
}

public partial class ReferenceType
{
    private IdentifierType documentIDField;
    public IdentifierType DocumentID{get{return documentIDField;}set{documentIDField = value;}}
}
public partial class DocumentReferenceType
{
    private IdentifierType idField;
    public IdentifierType ID{get{return this.idField;}set{this.idField = value;}}
}
public partial class IdentifierType
{
    private string identificationSchemeIDField;
    [System.Xml.Serialization.XmlAttributeAttribute(DataType = "normalizedString")]
    public string identificationSchemeID{get{return this.identificationSchemeIDField;}set{this.identificationSchemeIDField = value;}}
}