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ENM error: Object reference not set to an instance of an object

The following error is seen in Monitoring and Diagnostics component of Event Notification Module (ENM):

HandlingInstanceID: f155d374-d1d5-4c96-8ef8-f8dd3f4d726c
An exception of type 'System.NullReferenceException' occurred and was caught.
02/10/2016 16:05:50
Type : System.NullReferenceException, mscorlib, Version=, Culture=neutral, PublicKeyToken=b77a5c561934e089
Message : Object reference not set to an instance of an object.

Source : jrr2cetk
Help link : 
Data : System.Collections.ListDictionaryInternal
TargetSite : Boolean Evaluate()
HResult : -2147467261
Stack Trace :    at eSolutions.PowerLogic.AlarmGen.Expression.Evaluate() at eSolutions.PowerLogic.AlarmGen.Advanced.Prc_Advanced.AlarmStatus(Object& testValue, String& actCond)

Product Line
Event Notification Module (ENM)

Monitoring and Diagnostics (http://localhost:85)
Alarm Sentry (http://localhost:83)

A wrong expression when setting up Advanced Alarm in Alarm Sentry.
The Evaluate() function in ENM code evaluates the syntax and generates an error if it is not correct.  The error message indicates a problem when executing the Evaluate() function.

The syntax has to conform to C# and always return a boolean true or false:
  {return true;}
  {return false;}

Where the <tag> has to be replaced with an OPC tag.

An example:

if((Double)Source("localhost.ION.OpcDaServer.1.GEN_A.Gen_A_1/Eng Op Mode")==1)
  {return true;}
  {return false;}
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