Question

I’m trying to build a demonstrative OPC XML DA Server using C#. Developement is underway, but i am stuck on a serialization issue concerning arrays. Apparently, when I try to set ItemProperty.Value (which is of type Object) to any kind of array but byte[], i get this exception:

System.InvalidOperationException: The type <ArrayType> may not be used in this context.

Here’s the content of a sample GetProperties() method which triggers the exception:

[WebMethodAttribute()]
[
    SoapDocumentMethodAttribute
    (
        "http://opcfoundation.org/webservices/XMLDA/1.0/GetProperties", 
        RequestNamespace = "http://opcfoundation.org/webservices/XMLDA/1.0/", 
        ResponseNamespace = "http://opcfoundation.org/webservices/XMLDA/1.0/", 
        Use = SoapBindingUse.Literal, 
        ParameterStyle = SoapParameterStyle.Wrapped
    )
]
public override ReplyBase GetProperties
(
    [XmlElementAttribute("ItemIDs")] ItemIdentifier[] ItemIDs, 
    [XmlElementAttribute("PropertyNames")] XmlQualifiedName[] PropertyNames, 
    [XmlAttributeAttribute()] string LocaleID, 
    [XmlAttributeAttribute()] string ClientRequestHandle, 
    [XmlAttributeAttribute()] string ItemPath, 
    [XmlAttributeAttribute()] [DefaultValueAttribute(false)] bool ReturnAllProperties,
    [XmlAttributeAttribute()] [DefaultValueAttribute(false)] bool ReturnPropertyValues,
    [XmlAttributeAttribute()] [DefaultValueAttribute(false)] bool ReturnErrorText, 
    [XmlElementAttribute("PropertyLists")] out PropertyReplyList[] PropertyLists, 
    [XmlElementAttribute("Errors")] out OPCError[] Errors
)
{
    ReplyBase Response = new ReplyBase();
    Response.RcvTime = System.DateTime.Now;

    Response.RevisedLocaleID = LocaleID;
    Response.ClientRequestHandle = ClientRequestHandle;

    Errors = null;

    PropertyLists = new PropertyReplyList[1] { new PropertyReplyList() };
    PropertyLists[0].Properties = new ItemProperty[1] { new ItemProperty() };
    PropertyLists[0].Properties[0].Value = new short[2] { 3, 4 };

    Response.ReplyTime = System.DateTime.Now;
    return Response;
}

Full stack trace (italian)

System.Web.Services.Protocols.SoapException: Impossibile elaborare la richiesta. ---> System.InvalidOperationException: Errore durante la generazione del documento XML. ---> System.InvalidOperationException: Il tipo System.Int16[] può non essere utilizzato in questo contesto.
in System.Xml.Serialization.XmlSerializationWriter.WriteTypedPrimitive(String name, String ns, Object o, Boolean xsiType)
in Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationWriter1.Write2_Object(String n, String ns, Object o, Boolean isNullable, Boolean needType)
in Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationWriter1.Write16_ItemProperty(String n, String ns, ItemProperty o, Boolean isNullable, Boolean needType)
in Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationWriter1.Write19_PropertyReplyList(String n, String ns, PropertyReplyList o, Boolean isNullable, Boolean needType)
in Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationWriter1.Write26_GetPropertiesResponse(Object[] p)
in Microsoft.Xml.Serialization.GeneratedAssembly.ArrayOfObjectSerializer13.Serialize(Object objectToSerialize, XmlSerializationWriter writer)
in System.Xml.Serialization.XmlSerializer.Serialize(XmlWriter xmlWriter, Object o, XmlSerializerNamespaces namespaces, String encodingStyle, String id)
--- Fine della traccia dello stack dell'eccezione interna ---
in System.Xml.Serialization.XmlSerializer.Serialize(XmlWriter xmlWriter, Object o, XmlSerializerNamespaces namespaces, String encodingStyle, String id)
in System.Web.Services.Protocols.SoapServerProtocol.WriteReturns(Object[] returnValues, Stream outputStream)
in System.Web.Services.Protocols.WebServiceHandler.WriteReturns(Object[] returnValues)
in System.Web.Services.Protocols.WebServiceHandler.Invoke()
--- Fine della traccia dello stack dell'eccezione interna ---

I’ve already included into my project the service skeleton generated by wsdl.exe, and have absolutely no problems with non-array values. The target framework of my project is .NET 4.0 (but the same problem happens to 3.5).

My guess is that the method isn’t working well with the XmlIncludeAttribute() decorators in the service skeleton. Any clues on how to make it work?

Thank you for your time.

Edit: I’ve tried adding the XmlIncludeAttribute(typeof(int[])) decorator to the GetProperties() method (i guess decorators are not inheritable), and i’m not getting the exception. However, the client is unable to properly unserialize the underlying data. What I get from the client (written in VB.NET) after unserialization is an XmlNode reference instead of an int[]. The data is there, but it’s not properly unserialized.

Is this the expected behaviour? Might it be a client issue?

Answer 1

After a little more analysis of the produced XML I’ve found out the client required a specific namespace, which is the OPC web service default namespace, in order to properly deserialize data. I’ve updated the class definition of the web service implementation with the following decorators:

[WebService(Namespace = "http://opcfoundation.org/webservices/XMLDA/1.0/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[XmlInclude(typeof(short[]))]

The GetProperties() method now properly behaves, and the client is able to properly unserialize the array of Int16.