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POProcessing events from Event Log and react on a certain pattern (Rx?)
 

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    <p>I need to process events coming from the EventLog. This is easy using the <a href="http://msdn.microsoft.com/en-us/library/bb671202%28v=vs.90%29.aspx" rel="nofollow">EventLogWatcher</a> attaching to the EventRecordWritten event. However the query I am interested in (eventid == 299 || eventid == 500) provides more events than what I need. Here is an example of the stream</p> <pre><code>Event ID Correlation ID 299 1AD... (this is actually a guid) X1 500 1AD... X2 500 1AD... 500 1AD... 299 43B... Y1 299 EDB... Z1 500 43B... Y2 500 EDB... Z2 500 43B... 500 43B... </code></pre> <p>I am interested in the event 299 and the first event 500 that matches the correlation id of the 299 event. I marked them in the stream above, that's the output collection I am interested in: <code>[X1, X2, Y1, Y2, Z1, Z2]</code> which probably is a dictionary using the correlation id as a key and a tuple of <code>EventRecord</code> as the value</p> <pre><code>{ 1AD.., &lt;X1, X2&gt; } { 43B.., &lt;Y1, Y2&gt; } { EDB.., &lt;Z1, Z2&gt; } </code></pre> <p>In general events might come in order (299 and then 500) but in a high concurrency situation I foresee two 299 events coming together and then the 500 events so I don't want to rely on the order the events come. The correlation id is the key to correlate them (which is the first property of the event <code>eventRecord.Properties[0]</code>)</p> <p>I think this can be solved with a state machine but it would be interesting to see if anyone comes up with a solution with Rx represented by a query to an observable. That would keep the pattern matching logic in a single place.</p> <p><strong>UPDATE</strong>: here is the answer to the solution. Thanks Gideon it was exactly the starting point I needed!</p> <pre><code>var pairs = events .Where(e299 =&gt; e299.EventArgs.EventRecord.Id == 299) .SelectMany(e299 =&gt; events.Where(e500 =&gt; e500.EventArgs.EventRecord.Id == 500 &amp;&amp; e299.EventArgs.EventRecord.Properties[0].Value.ToString() == e500.EventArgs.EventRecord.Properties[0].Value.ToString()) .Take(1), (e299, e500) =&gt; new { First = e299, Second = e500 }); </code></pre> <p>Thanks in advance, Matias</p>
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