StreamInsight: Obscure LINQ error - Stream other than apply input stream is cannot be referenced inside apply branch

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Another little LINQ error you might encounter from time to time.  Ran into this yesterday while building out some queries, and figured it was worth a quick post.  Starting with a basic stream, I needed to group by a set of fields in the stream and calculate some basic aggregates.

Code Snippet

  1. // This query calculates the sum of all sensor values
  2. // for each sensor
  3. // for each 5 seconds worth of data.            
  4. var query = from e in inputStream
  5.     group e by e.SensorId into sensorGroups
  6.             from window in inputStream.TumblingWindow(
  7.         TimeSpan.FromSeconds(5),
  8.         HoppingWindowOutputPolicy.ClipToWindowEnd)
  9.     select new
  10.     {
  11.         SensorId = sensorGroups.Key,
  12.         Sum = window.Sum(e => e.Value)
  13.     };

Running this throws the error:

 Microsoft.ComplexEventProcessing.Linq.QueryGenerationException was unhandled by user code  Message=Stream other than apply input stream is cannot be referenced inside apply branch. The following expression is not supported: 'sensorGroups => CreateAdapterStream("input", StreamInsight.Samples.Adapters.SimpleTextFileReader.TextFileReaderFactory, value(StreamInsight.Samples.Adapters.SimpleTextFileReader.TextFileReaderConfig), Point, value(Microsoft.ComplexEventProcessing.Linq.CepStreamCreationContext)).TumblingWindow(FromSeconds(5), HoppingWindowOutputPolicy.ClipToWindowEnd)'.

See the subtle yet annoyingly obvious after the fact mistake I made?  I grouped by sensorGroups, but windowed over inputStream.  Fix this to use the same stream for the window and the group resolves the error.

Code Snippet

  1. var query = from e in inputStream
  2.     group e by e.SensorId into sensorGroups
  3.             from window in sensorGroups.TumblingWindow(
  4.         TimeSpan.FromSeconds(5),
  5.         HoppingWindowOutputPolicy.ClipToWindowEnd)
  6.     select new
  7.     {
  8.         SensorId = sensorGroups.Key,
  9.         Sum = window.Sum(e => e.Value)
  10.     };