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Object reference not set to an instance of an object

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14 May 2021 15:43 - 14 May 2021 15:44 #9683 by support
Hello.

1. Your code should first test whether
e.EventData.FieldResults[UAFilterElements.SimpleAttribute("ns=2; i = 1", "/TEXT06")].Exception
is null or not.

If it is not-null, there has been an error related to the particular attribute, and from the .Exception you will get an idea of what the error was.
If it is not-null, then .Value is null, and .Value.ToString() will throw a NullReferenceException, which is likely the cause of your problem. If you just want to ignore the error (not recommended), you could write .Value?.ToString(), which will mean your TEXT06 will also become null in case of an error.

2. Is
"ns=2; i = 1"
the actual precise code, including the space characters, or do the space characters come e.g. from copy-paste publishing to the forums? If the space characters are truly in your code, then this node Id is most likely incorrect (which in turn may be cause the error in #1 or contribute to it). If the intent was to specify a node Id in namespace index 2 with integer Id value 1, then it should be
"ns=2;i=1"
The "i=" must immediately follow the semicolon. Otherwise, anything after the semicolon, if it does not start with "i=", "g=", "s=" or "b=", will be interpreted as a string identifier, so what I see in your post is actually equivalent to "ns=2;s= i = 1".

Best regards
Last edit: 14 May 2021 15:44 by support.
The following user(s) said Thank You: AdiROC

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14 May 2021 13:35 #9680 by AdiROC
Hello

I use an Alarms and Conditions Client.

When I initialize an Eventsubscription, I subscribe to a custom node on the server(TEXT06):
{
            UAEndpointDescriptor endpointDescriptor =
            "opc.tcp://192.168.1.1:4840/";
 
            // Instantiate the client object and hook events
            var client = new EasyUAClient();
            client.EventNotification += client_EventNotification;
 
            subscriptionID = client.SubscribeEvent(
                endpointDescriptor,
                UAObjectIds.Server,
                1000,
                new UAAttributeFieldCollection
                {
                    UAFilterElements.SimpleAttribute("ns=2; i = 1", "/TEXT06"),
                    UABaseEventObject.Operands.Time
                });
        }

Depending on the event the node can also be empty.

If the node is empty, I get an error, even if I use a try/catch statement:
static async void client_EventNotification(object sender, EasyUAEventNotificationEventArgs e)
        {
            string TEXT06;
            try
            {
                TEXT06 = e.EventData.FieldResults[UAFilterElements.SimpleAttribute("ns=2; i = 1", "/TEXT06")].Value.ToString();//Ausgefallen
            }
            catch
            {
                //error
            }
        }
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