高翔 / ECS-ZJHX · Files

using Opc.Ua;
using Opc.Ua.Client;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace OpcUaHelper
{
    /// <summary>
    /// 辅助类
    /// </summary>
    public class FormUtils
    {
        /// <summary>
        /// Gets the display text for the access level attribute.
        /// </summary>
        /// <param name="accessLevel">The access level.</param>
        /// <returns>The access level formatted as a string.</returns>
        private static string GetAccessLevelDisplayText( byte accessLevel )
        {
            StringBuilder buffer = new StringBuilder( );

            if (accessLevel == AccessLevels.None)
            {
                buffer.Append( "None" );
            }

            if ((accessLevel & AccessLevels.CurrentRead) == AccessLevels.CurrentRead)
            {
                buffer.Append( "Read" );
            }

            if ((accessLevel & AccessLevels.CurrentWrite) == AccessLevels.CurrentWrite)
            {
                if (buffer.Length > 0)
                {
                    buffer.Append( " | " );
                }

                buffer.Append( "Write" );
            }

            if ((accessLevel & AccessLevels.HistoryRead) == AccessLevels.HistoryRead)
            {
                if (buffer.Length > 0)
                {
                    buffer.Append( " | " );
                }

                buffer.Append( "HistoryRead" );
            }

            if ((accessLevel & AccessLevels.HistoryWrite) == AccessLevels.HistoryWrite)
            {
                if (buffer.Length > 0)
                {
                    buffer.Append( " | " );
                }

                buffer.Append( "HistoryWrite" );
            }

            if ((accessLevel & AccessLevels.SemanticChange) == AccessLevels.SemanticChange)
            {
                if (buffer.Length > 0)
                {
                    buffer.Append( " | " );
                }

                buffer.Append( "SemanticChange" );
            }

            return buffer.ToString( );
        }

        /// <summary>
        /// Gets the display text for the event notifier attribute.
        /// </summary>
        /// <param name="eventNotifier">The event notifier.</param>
        /// <returns>The event notifier formatted as a string.</returns>
        private static string GetEventNotifierDisplayText( byte eventNotifier )
        {
            StringBuilder buffer = new StringBuilder( );

            if (eventNotifier == EventNotifiers.None)
            {
                buffer.Append( "None" );
            }

            if ((eventNotifier & EventNotifiers.SubscribeToEvents) == EventNotifiers.SubscribeToEvents)
            {
                buffer.Append( "Subscribe" );
            }

            if ((eventNotifier & EventNotifiers.HistoryRead) == EventNotifiers.HistoryRead)
            {
                if (buffer.Length > 0)
                {
                    buffer.Append( " | " );
                }

                buffer.Append( "HistoryRead" );
            }

            if ((eventNotifier & EventNotifiers.HistoryWrite) == EventNotifiers.HistoryWrite)
            {
                if (buffer.Length > 0)
                {
                    buffer.Append( " | " );
                }

                buffer.Append( "HistoryWrite" );
            }

            return buffer.ToString( );
        }

        /// <summary>
        /// Gets the display text for the value rank attribute.
        /// </summary>
        /// <param name="valueRank">The value rank.</param>
        /// <returns>The value rank formatted as a string.</returns>
        private static string GetValueRankDisplayText( int valueRank )
        {
            switch (valueRank)
            {
                case ValueRanks.Any: return "Any";
                case ValueRanks.Scalar: return "Scalar";
                case ValueRanks.ScalarOrOneDimension: return "ScalarOrOneDimension";
                case ValueRanks.OneOrMoreDimensions: return "OneOrMoreDimensions";
                case ValueRanks.OneDimension: return "OneDimension";
                case ValueRanks.TwoDimensions: return "TwoDimensions";
            }

            return valueRank.ToString( );
        }

        /// <summary>
        /// Gets the display text for the specified attribute.
        /// </summary>
        /// <param name="session">The currently active session.</param>
        /// <param name="attributeId">The id of the attribute.</param>
        /// <param name="value">The value of the attribute.</param>
        /// <returns>The attribute formatted as a string.</returns>
        public static string GetAttributeDisplayText( Session session, uint attributeId, Variant value )
        {
            if (value == Variant.Null)
            {
                return String.Empty;
            }

            switch (attributeId)
            {
                case Attributes.AccessLevel:
                case Attributes.UserAccessLevel:
                    {
                        byte? field = value.Value as byte?;

                        if (field != null)
                        {
                            return GetAccessLevelDisplayText( field.Value );
                        }

                        break;
                    }

                case Attributes.EventNotifier:
                    {
                        byte? field = value.Value as byte?;

                        if (field != null)
                        {
                            return GetEventNotifierDisplayText( field.Value );
                        }

                        break;
                    }

                case Attributes.DataType:
                    {
                        return session.NodeCache.GetDisplayText( value.Value as NodeId );
                    }

                case Attributes.ValueRank:
                    {
                        int? field = value.Value as int?;

                        if (field != null)
                        {
                            return GetValueRankDisplayText( field.Value );
                        }

                        break;
                    }

                case Attributes.NodeClass:
                    {
                        int? field = value.Value as int?;

                        if (field != null)
                        {
                            return ((NodeClass)field.Value).ToString( );
                        }

                        break;
                    }

                case Attributes.NodeId:
                    {
                        NodeId field = value.Value as NodeId;

                        if (!NodeId.IsNull( field ))
                        {
                            return field.ToString( );
                        }

                        return "Null";
                    }
            }

            // check for byte strings.
            if (value.Value is byte[])
            {
                return Utils.ToHexString( value.Value as byte[] );
            }

            // use default format.
            return value.ToString( );
        }

        /// <summary>
        /// Discovers the servers on the local machine.
        /// </summary>
        /// <param name="configuration">The configuration.</param>
        /// <returns>A list of server urls.</returns>
        public static IList<string> DiscoverServers( ApplicationConfiguration configuration )
        {
            List<string> serverUrls = new List<string>( );

            // set a short timeout because this is happening in the drop down event.
            EndpointConfiguration endpointConfiguration = EndpointConfiguration.Create( configuration );
            endpointConfiguration.OperationTimeout = 5000;

            // Connect to the local discovery server and find the available servers.
            using (DiscoveryClient client = DiscoveryClient.Create( new Uri( "opc.tcp://localhost:4840" ), endpointConfiguration ))
            {
                ApplicationDescriptionCollection servers = client.FindServers( null );

                // populate the drop down list with the discovery URLs for the available servers.
                for (int ii = 0; ii < servers.Count; ii++)
                {
                    if (servers[ii].ApplicationType == ApplicationType.DiscoveryServer)
                    {
                        continue;
                    }

                    for (int jj = 0; jj < servers[ii].DiscoveryUrls.Count; jj++)
                    {
                        string discoveryUrl = servers[ii].DiscoveryUrls[jj];

                        // Many servers will use the '/discovery' suffix for the discovery endpoint.
                        // The URL without this prefix should be the base URL for the server. 
                        if (discoveryUrl.EndsWith( "/discovery" ))
                        {
                            discoveryUrl = discoveryUrl.Substring( 0, discoveryUrl.Length - "/discovery".Length );
                        }

                        // ensure duplicates do not get added.
                        if (!serverUrls.Contains( discoveryUrl ))
                        {
                            serverUrls.Add( discoveryUrl );
                        }
                    }
                }
            }

            return serverUrls;
        }

        /// <summary>
        /// Finds the endpoint that best matches the current settings.
        /// </summary>
        /// <param name="discoveryUrl">The discovery URL.</param>
        /// <param name="useSecurity">if set to <c>true</c> select an endpoint that uses security.</param>
        /// <returns>The best available endpoint.</returns>
        public static EndpointDescription SelectEndpoint( string discoveryUrl, bool useSecurity )
        {
            // needs to add the '/discovery' back onto non-UA TCP URLs.
            if (!discoveryUrl.StartsWith( Utils.UriSchemeOpcTcp ))
            {
                if (!discoveryUrl.EndsWith( "/discovery" ))
                {
                    discoveryUrl += "/discovery";
                }
            }

            // parse the selected URL.
            Uri uri = new Uri( discoveryUrl );

            // set a short timeout because this is happening in the drop down event.
            EndpointConfiguration configuration = EndpointConfiguration.Create( );
            configuration.OperationTimeout = 5000;

            EndpointDescription selectedEndpoint = null;

            // Connect to the server's discovery endpoint and find the available configuration.
            using (DiscoveryClient client = DiscoveryClient.Create( uri, configuration ))
            {
                EndpointDescriptionCollection endpoints = client.GetEndpoints( null );

                // select the best endpoint to use based on the selected URL and the UseSecurity checkbox. 
                for (int ii = 0; ii < endpoints.Count; ii++)
                {
                    EndpointDescription endpoint = endpoints[ii];

                    // check for a match on the URL scheme.
                    if (endpoint.EndpointUrl.StartsWith( uri.Scheme ))
                    {
                        // check if security was requested.
                        if (useSecurity)
                        {
                            if (endpoint.SecurityMode == MessageSecurityMode.None)
                            {
                                continue;
                            }
                        }
                        else
                        {
                            if (endpoint.SecurityMode != MessageSecurityMode.None)
                            {
                                continue;
                            }
                        }

                        // pick the first available endpoint by default.
                        if (selectedEndpoint == null)
                        {
                            selectedEndpoint = endpoint;
                        }

                        // The security level is a relative measure assigned by the server to the 
                        // endpoints that it returns. Clients should always pick the highest level
                        // unless they have a reason not too.
                        if (endpoint.SecurityLevel > selectedEndpoint.SecurityLevel)
                        {
                            selectedEndpoint = endpoint;
                        }
                    }
                }

                // pick the first available endpoint by default.
                if (selectedEndpoint == null && endpoints.Count > 0)
                {
                    selectedEndpoint = endpoints[0];
                }
            }

            // if a server is behind a firewall it may return URLs that are not accessible to the client.
            // This problem can be avoided by assuming that the domain in the URL used to call 
            // GetEndpoints can be used to access any of the endpoints. This code makes that conversion.
            // Note that the conversion only makes sense if discovery uses the same protocol as the endpoint.

            Uri endpointUrl = Utils.ParseUri( selectedEndpoint.EndpointUrl );

            if (endpointUrl != null && endpointUrl.Scheme == uri.Scheme)
            {
                UriBuilder builder = new UriBuilder( endpointUrl );
                builder.Host = uri.DnsSafeHost;
                builder.Port = uri.Port;
                selectedEndpoint.EndpointUrl = builder.ToString( );
            }

            // return the selected endpoint.
            return selectedEndpoint;
        }

        /// <summary>
        /// Browses the address space and returns the references found.
        /// </summary>
        /// <param name="session">The session.</param>
        /// <param name="nodesToBrowse">The set of browse operations to perform.</param>
        /// <param name="throwOnError">if set to <c>true</c> a exception will be thrown on an error.</param>
        /// <returns>
        /// The references found. Null if an error occurred.
        /// </returns>
        public static ReferenceDescriptionCollection Browse( Session session, BrowseDescriptionCollection nodesToBrowse, bool throwOnError )
        {
            try
            {
                ReferenceDescriptionCollection references = new ReferenceDescriptionCollection( );
                BrowseDescriptionCollection unprocessedOperations = new BrowseDescriptionCollection( );

                while (nodesToBrowse.Count > 0)
                {
                    // start the browse operation.
                    BrowseResultCollection results = null;
                    DiagnosticInfoCollection diagnosticInfos = null;

                    session.Browse(
                        null,
                        null,
                        0,
                        nodesToBrowse,
                        out results,
                        out diagnosticInfos );

                    ClientBase.ValidateResponse( results, nodesToBrowse );
                    ClientBase.ValidateDiagnosticInfos( diagnosticInfos, nodesToBrowse );

                    ByteStringCollection continuationPoints = new ByteStringCollection( );

                    for (int ii = 0; ii < nodesToBrowse.Count; ii++)
                    {
                        // check for error.
                        if (StatusCode.IsBad( results[ii].StatusCode ))
                        {
                            // this error indicates that the server does not have enough simultaneously active 
                            // continuation points. This request will need to be resent after the other operations
                            // have been completed and their continuation points released.
                            if (results[ii].StatusCode == StatusCodes.BadNoContinuationPoints)
                            {
                                unprocessedOperations.Add( nodesToBrowse[ii] );
                            }

                            continue;
                        }

                        // check if all references have been fetched.
                        if (results[ii].References.Count == 0)
                        {
                            continue;
                        }

                        // save results.
                        references.AddRange( results[ii].References );

                        // check for continuation point.
                        if (results[ii].ContinuationPoint != null)
                        {
                            continuationPoints.Add( results[ii].ContinuationPoint );
                        }
                    }

                    // process continuation points.
                    ByteStringCollection revisedContiuationPoints = new ByteStringCollection( );

                    while (continuationPoints.Count > 0)
                    {
                        // continue browse operation.
                        session.BrowseNext(
                            null,
                            true,
                            continuationPoints,
                            out results,
                            out diagnosticInfos );

                        ClientBase.ValidateResponse( results, continuationPoints );
                        ClientBase.ValidateDiagnosticInfos( diagnosticInfos, continuationPoints );

                        for (int ii = 0; ii < continuationPoints.Count; ii++)
                        {
                            // check for error.
                            if (StatusCode.IsBad( results[ii].StatusCode ))
                            {
                                continue;
                            }

                            // check if all references have been fetched.
                            if (results[ii].References.Count == 0)
                            {
                                continue;
                            }

                            // save results.
                            references.AddRange( results[ii].References );

                            // check for continuation point.
                            if (results[ii].ContinuationPoint != null)
                            {
                                revisedContiuationPoints.Add( results[ii].ContinuationPoint );
                            }
                        }

                        // check if browsing must continue;
                        revisedContiuationPoints = continuationPoints;
                    }

                    // check if unprocessed results exist.
                    nodesToBrowse = unprocessedOperations;
                }

                // return complete list.
                return references;
            }
            catch (Exception exception)
            {
                if (throwOnError)
                {
                    throw new ServiceResultException( exception, StatusCodes.BadUnexpectedError );
                }

                return null;
            }
        }

        /// <summary>
        /// Finds the type of the event for the notification.
        /// </summary>
        /// <param name="monitoredItem">The monitored item.</param>
        /// <param name="notification">The notification.</param>
        /// <returns>The NodeId of the EventType.</returns>
        public static NodeId FindEventType( MonitoredItem monitoredItem, EventFieldList notification )
        {
            EventFilter filter = monitoredItem.Status.Filter as EventFilter;

            if (filter != null)
            {
                for (int ii = 0; ii < filter.SelectClauses.Count; ii++)
                {
                    SimpleAttributeOperand clause = filter.SelectClauses[ii];

                    if (clause.BrowsePath.Count == 1 && clause.BrowsePath[0] == BrowseNames.EventType)
                    {
                        return notification.EventFields[ii].Value as NodeId;
                    }
                }
            }

            return null;
        }

        /// <summary>
        /// Browses the address space and returns the references found.
        /// </summary>
        /// <param name="session">The session.</param>
        /// <param name="nodeToBrowse">The NodeId for the starting node.</param>
        /// <param name="throwOnError">if set to <c>true</c> a exception will be thrown on an error.</param>
        /// <returns>
        /// The references found. Null if an error occurred.
        /// </returns>
        public static ReferenceDescriptionCollection Browse( Session session, BrowseDescription nodeToBrowse, bool throwOnError )
        {
            try
            {
                ReferenceDescriptionCollection references = new ReferenceDescriptionCollection( );

                // construct browse request.
                BrowseDescriptionCollection nodesToBrowse = new BrowseDescriptionCollection( );
                nodesToBrowse.Add( nodeToBrowse );

                // start the browse operation.
                BrowseResultCollection results = null;
                DiagnosticInfoCollection diagnosticInfos = null;

                session.Browse(
                    null,
                    null,
                    0,
                    nodesToBrowse,
                    out results,
                    out diagnosticInfos );

                ClientBase.ValidateResponse( results, nodesToBrowse );
                ClientBase.ValidateDiagnosticInfos( diagnosticInfos, nodesToBrowse );

                do
                {
                    // check for error.
                    if (StatusCode.IsBad( results[0].StatusCode ))
                    {
                        throw new ServiceResultException( results[0].StatusCode );
                    }

                    // process results.
                    for (int ii = 0; ii < results[0].References.Count; ii++)
                    {
                        references.Add( results[0].References[ii] );
                    }

                    // check if all references have been fetched.
                    if (results[0].References.Count == 0 || results[0].ContinuationPoint == null)
                    {
                        break;
                    }

                    // continue browse operation.
                    ByteStringCollection continuationPoints = new ByteStringCollection( );
                    continuationPoints.Add( results[0].ContinuationPoint );

                    session.BrowseNext(
                        null,
                        false,
                        continuationPoints,
                        out results,
                        out diagnosticInfos );

                    ClientBase.ValidateResponse( results, continuationPoints );
                    ClientBase.ValidateDiagnosticInfos( diagnosticInfos, continuationPoints );
                }
                while (true);

                //return complete list.
                return references;
            }
            catch (Exception exception)
            {
                if (throwOnError)
                {
                    throw new ServiceResultException( exception, StatusCodes.BadUnexpectedError );
                }

                return null;
            }
        }

        /// <summary>
        /// Browses the address space and returns all of the supertypes of the specified type node.
        /// </summary>
        /// <param name="session">The session.</param>
        /// <param name="typeId">The NodeId for a type node in the address space.</param>
        /// <param name="throwOnError">if set to <c>true</c> a exception will be thrown on an error.</param>
        /// <returns>
        /// The references found. Null if an error occurred.
        /// </returns>
        public static ReferenceDescriptionCollection BrowseSuperTypes( Session session, NodeId typeId, bool throwOnError )
        {
            ReferenceDescriptionCollection supertypes = new ReferenceDescriptionCollection( );

            try
            {
                // find all of the children of the field.
                BrowseDescription nodeToBrowse = new BrowseDescription( );

                nodeToBrowse.NodeId = typeId;
                nodeToBrowse.BrowseDirection = BrowseDirection.Inverse;
                nodeToBrowse.ReferenceTypeId = ReferenceTypeIds.HasSubtype;
                nodeToBrowse.IncludeSubtypes = false; // more efficient to use IncludeSubtypes=False when possible.
                nodeToBrowse.NodeClassMask = 0; // the HasSubtype reference already restricts the targets to Types. 
                nodeToBrowse.ResultMask = (uint)BrowseResultMask.All;

                ReferenceDescriptionCollection references = Browse( session, nodeToBrowse, throwOnError );

                while (references != null && references.Count > 0)
                {
                    // should never be more than one supertype.
                    supertypes.Add( references[0] );

                    // only follow references within this server.
                    if (references[0].NodeId.IsAbsolute)
                    {
                        break;
                    }

                    // get the references for the next level up.
                    nodeToBrowse.NodeId = (NodeId)references[0].NodeId;
                    references = Browse( session, nodeToBrowse, throwOnError );
                }

                // return complete list.
                return supertypes;
            }
            catch (Exception exception)
            {
                if (throwOnError)
                {
                    throw new ServiceResultException( exception, StatusCodes.BadUnexpectedError );
                }

                return null;
            }
        }

        /// <summary>
        /// Constructs an event object from a notification.
        /// </summary>
        /// <param name="session">The session.</param>
        /// <param name="monitoredItem">The monitored item that produced the notification.</param>
        /// <param name="notification">The notification.</param>
        /// <param name="knownEventTypes">The known event types.</param>
        /// <param name="eventTypeMappings">Mapping between event types and known event types.</param>
        /// <returns>
        /// The event object. Null if the notification is not a valid event type.
        /// </returns>
        public static BaseEventState ConstructEvent(
            Session session,
            MonitoredItem monitoredItem,
            EventFieldList notification,
            Dictionary<NodeId, Type> knownEventTypes,
            Dictionary<NodeId, NodeId> eventTypeMappings )
        {
            // find the event type.
            NodeId eventTypeId = FindEventType( monitoredItem, notification );

            if (eventTypeId == null)
            {
                return null;
            }

            // look up the known event type.
            Type knownType = null;
            NodeId knownTypeId = null;

            if (eventTypeMappings.TryGetValue( eventTypeId, out knownTypeId ))
            {
                knownType = knownEventTypes[knownTypeId];
            }

            // try again.
            if (knownType == null)
            {
                if (knownEventTypes.TryGetValue( eventTypeId, out knownType ))
                {
                    knownTypeId = eventTypeId;
                    eventTypeMappings.Add( eventTypeId, eventTypeId );
                }
            }

            // try mapping it to a known type.
            if (knownType == null)
            {
                // browse for the supertypes of the event type.
                ReferenceDescriptionCollection supertypes = FormUtils.BrowseSuperTypes( session, eventTypeId, false );

                // can't do anything with unknown types.
                if (supertypes == null)
                {
                    return null;
                }

                // find the first supertype that matches a known event type.
                for (int ii = 0; ii < supertypes.Count; ii++)
                {
                    NodeId superTypeId = (NodeId)supertypes[ii].NodeId;

                    if (knownEventTypes.TryGetValue( superTypeId, out knownType ))
                    {
                        knownTypeId = superTypeId;
                        eventTypeMappings.Add( eventTypeId, superTypeId );
                    }

                    if (knownTypeId != null)
                    {
                        break;
                    }
                }

                // can't do anything with unknown types.
                if (knownTypeId == null)
                {
                    return null;
                }
            }

            // construct the event based on the known event type.
            BaseEventState e = (BaseEventState)Activator.CreateInstance( knownType, new object[] { (NodeState)null } );

            // get the filter which defines the contents of the notification.
            EventFilter filter = monitoredItem.Status.Filter as EventFilter;

            // initialize the event with the values in the notification.
            e.Update( session.SystemContext, filter.SelectClauses, notification );

            // save the orginal notification.
            e.Handle = notification;

            return e;
        }

        /// <summary>
        /// Returns the node ids for a set of relative paths.
        /// </summary>
        /// <param name="session">An open session with the server to use.</param>
        /// <param name="startNodeId">The starting node for the relative paths.</param>
        /// <param name="namespacesUris">The namespace URIs referenced by the relative paths.</param>
        /// <param name="relativePaths">The relative paths.</param>
        /// <returns>A collection of local nodes.</returns>
        public static List<NodeId> TranslateBrowsePaths(
            Session session,
            NodeId startNodeId,
            NamespaceTable namespacesUris,
            params string[] relativePaths )
        {
            // build the list of browse paths to follow by parsing the relative paths.
            BrowsePathCollection browsePaths = new BrowsePathCollection( );

            if (relativePaths != null)
            {
                for (int ii = 0; ii < relativePaths.Length; ii++)
                {
                    BrowsePath browsePath = new BrowsePath( );

                    // The relative paths used indexes in the namespacesUris table. These must be 
                    // converted to indexes used by the server. An error occurs if the relative path
                    // refers to a namespaceUri that the server does not recognize.

                    // The relative paths may refer to ReferenceType by their BrowseName. The TypeTree object
                    // allows the parser to look up the server's NodeId for the ReferenceType.

                    browsePath.RelativePath = RelativePath.Parse(
                        relativePaths[ii],
                        session.TypeTree,
                        namespacesUris,
                        session.NamespaceUris );

                    browsePath.StartingNode = startNodeId;

                    browsePaths.Add( browsePath );
                }
            }

            // make the call to the server.
            BrowsePathResultCollection results;
            DiagnosticInfoCollection diagnosticInfos;

            ResponseHeader responseHeader = session.TranslateBrowsePathsToNodeIds(
                null,
                browsePaths,
                out results,
                out diagnosticInfos );

            // ensure that the server returned valid results.
            Session.ValidateResponse( results, browsePaths );
            Session.ValidateDiagnosticInfos( diagnosticInfos, browsePaths );

            // collect the list of node ids found.
            List<NodeId> nodes = new List<NodeId>( );

            for (int ii = 0; ii < results.Count; ii++)
            {
                // check if the start node actually exists.
                if (StatusCode.IsBad( results[ii].StatusCode ))
                {
                    nodes.Add( null );
                    continue;
                }

                // an empty list is returned if no node was found.
                if (results[ii].Targets.Count == 0)
                {
                    nodes.Add( null );
                    continue;
                }

                // Multiple matches are possible, however, the node that matches the type model is the
                // one we are interested in here. The rest can be ignored.
                BrowsePathTarget target = results[ii].Targets[0];

                if (target.RemainingPathIndex != UInt32.MaxValue)
                {
                    nodes.Add( null );
                    continue;
                }

                // The targetId is an ExpandedNodeId because it could be node in another server. 
                // The ToNodeId function is used to convert a local NodeId stored in a ExpandedNodeId to a NodeId.
                nodes.Add( ExpandedNodeId.ToNodeId( target.TargetId, session.NamespaceUris ) );
            }

            // return whatever was found.
            return nodes;
        }

        /// <summary>
        /// Collects the fields for the type.
        /// </summary>
        /// <param name="session">The session.</param>
        /// <param name="fields">The fields.</param>
        /// <param name="fieldNodeIds">The node id for the declaration of the field.</param>
        public static void CollectFieldsForType( Session session, NodeId typeId, SimpleAttributeOperandCollection fields, List<NodeId> fieldNodeIds )
        {
            // get the supertypes.
            ReferenceDescriptionCollection supertypes = FormUtils.BrowseSuperTypes( session, typeId, false );

            if (supertypes == null)
            {
                return;
            }

            // process the types starting from the top of the tree.
            Dictionary<NodeId, QualifiedNameCollection> foundNodes = new Dictionary<NodeId, QualifiedNameCollection>( );
            QualifiedNameCollection parentPath = new QualifiedNameCollection( );

            for (int ii = supertypes.Count - 1; ii >= 0; ii--)
            {
                CollectFields( session, (NodeId)supertypes[ii].NodeId, parentPath, fields, fieldNodeIds, foundNodes );
            }

            // collect the fields for the selected type.
            CollectFields( session, typeId, parentPath, fields, fieldNodeIds, foundNodes );
        }

        /// <summary>
        /// Collects the fields for the instance.
        /// </summary>
        /// <param name="session">The session.</param>
        /// <param name="fields">The fields.</param>
        /// <param name="instanceId">The node id for the declaration of the field.</param>
        public static void CollectFieldsForInstance( Session session, NodeId instanceId, SimpleAttributeOperandCollection fields, List<NodeId> fieldNodeIds )
        {
            Dictionary<NodeId, QualifiedNameCollection> foundNodes = new Dictionary<NodeId, QualifiedNameCollection>( );
            QualifiedNameCollection parentPath = new QualifiedNameCollection( );
            CollectFields( session, instanceId, parentPath, fields, fieldNodeIds, foundNodes );
        }

        /// <summary>
        /// Collects the fields for the instance node.
        /// </summary>
        /// <param name="session">The session.</param>
        /// <param name="nodeId">The node id.</param>
        /// <param name="parentPath">The parent path.</param>
        /// <param name="fields">The event fields.</param>
        /// <param name="fieldNodeIds">The node id for the declaration of the field.</param>
        /// <param name="foundNodes">The table of found nodes.</param>
        private static void CollectFields(
            Session session,
            NodeId nodeId,
            QualifiedNameCollection parentPath,
            SimpleAttributeOperandCollection fields,
            List<NodeId> fieldNodeIds,
            Dictionary<NodeId, QualifiedNameCollection> foundNodes )
        {
            // find all of the children of the field.
            BrowseDescription nodeToBrowse = new BrowseDescription( );

            nodeToBrowse.NodeId = nodeId;
            nodeToBrowse.BrowseDirection = BrowseDirection.Forward;
            nodeToBrowse.ReferenceTypeId = ReferenceTypeIds.Aggregates;
            nodeToBrowse.IncludeSubtypes = true;
            nodeToBrowse.NodeClassMask = (uint)(NodeClass.Object | NodeClass.Variable);
            nodeToBrowse.ResultMask = (uint)BrowseResultMask.All;

            ReferenceDescriptionCollection children = FormUtils.Browse( session, nodeToBrowse, false );

            if (children == null)
            {
                return;
            }

            // process the children.
            for (int ii = 0; ii < children.Count; ii++)
            {
                ReferenceDescription child = children[ii];

                if (child.NodeId.IsAbsolute)
                {
                    continue;
                }

                // construct browse path.
                QualifiedNameCollection browsePath = new QualifiedNameCollection( parentPath );
                browsePath.Add( child.BrowseName );

                // check if the browse path is already in the list.
                int index = ContainsPath( fields, browsePath );

                if (index < 0)
                {
                    SimpleAttributeOperand field = new SimpleAttributeOperand( );

                    field.TypeDefinitionId = ObjectTypeIds.BaseEventType;
                    field.BrowsePath = browsePath;
                    field.AttributeId = (child.NodeClass == NodeClass.Variable) ? Attributes.Value : Attributes.NodeId;

                    fields.Add( field );
                    fieldNodeIds.Add( (NodeId)child.NodeId );
                }

                // recusively find all of the children.
                NodeId targetId = (NodeId)child.NodeId;

                // need to guard against loops.
                if (!foundNodes.ContainsKey( targetId ))
                {
                    foundNodes.Add( targetId, browsePath );
                    CollectFields( session, (NodeId)child.NodeId, browsePath, fields, fieldNodeIds, foundNodes );
                }
            }
        }

        /// <summary>
        /// Determines whether the specified select clause contains the browse path.
        /// </summary>
        /// <param name="selectClause">The select clause.</param>
        /// <param name="browsePath">The browse path.</param>
        /// <returns>
        /// 	<c>true</c> if the specified select clause contains path; otherwise, <c>false</c>.
        /// </returns>
        private static int ContainsPath( SimpleAttributeOperandCollection selectClause, QualifiedNameCollection browsePath )
        {
            for (int ii = 0; ii < selectClause.Count; ii++)
            {
                SimpleAttributeOperand field = selectClause[ii];

                if (field.BrowsePath.Count != browsePath.Count)
                {
                    continue;
                }

                bool match = true;

                for (int jj = 0; jj < field.BrowsePath.Count; jj++)
                {
                    if (field.BrowsePath[jj] != browsePath[jj])
                    {
                        match = false;
                        break;
                    }
                }

                if (match)
                {
                    return ii;
                }
            }

            return -1;
        }
    }
}