Why was exception not caught within the closure? [duplicate]

Question

Here is the code example:

    public IList<LogEntry> ReadLogs(Guid id, string name)
    {
        var logs = this.RetrieveLogs(id, name);

        if (logs != null)
        {
            foreach (LogEvent logEvent in logs)
            {
                // bla, bla, bla
            }
        }

        return logEntries;
    }

    private IEnumerable<LogEvent> RetrieveLogs(Guid id, string name)
    {
        try
        {
            FilterCriteria filterCriteria = CreateFilterCriteria();
            return (from log in this.loggingProvider.ReadLogs(filterCriteria, 1)
                    where log.ParticipantObjects[0].ParticipantObjectId ==  id.ToString() 
                     && log.LogEventParameters[0].Value == name
                    orderby log.Timestamp.ToLocalTime() descending
                    select log).AsEnumerable();
        }
        catch (Exception ex)
        {
            this.tracer.Write(Category.Error, ex, "Error");
            return null;
        }
    }

Now, if there was an exception inside of loggingProvider.ReadLogs() method, it will be caught and traced. But if, for example, there is no ParticipantObjects[0], this exception won't be caught and traced here. It seems that it has something to do with lambda expressions and closures.

What is the explanation?

Answer 1

I have said it before and I will undoubtedly say it again: the most important thing to know about a query is that a query is a question, not the answer to the question.

The query object you've built does not execute merely by building it. You are building the question. The question does not actually get asked and answered until you execute the query. And the execution is outside the try block.

Answer 2

loggingProvider.ReadLogs is executed inside RetrieveLogs. But all the other lambdas are executed only when you iterate over them in the outer this.ReadLogs.

So the exception is thrown in your upper method, and so cannot be catched inside RetrieveLogs.

To avoid this, change AsEnumerable to ToList() or ToArray() to ensure the query is actually executed before returning.

Answer 3

An IEnumerable result from a LINQ query is only evaluated when it is enumerated.

In order for the Exception to be thrown where you expect, you need to enumerate the LINQ query result IEnumerable using one of the relevant extension methods like ToList().

As a side-note (borrowing from Rene Vogt) you could avoid the Exception being thrown by writing your LINQ as follows:

return this.loggingProvider.ReadLogs(filterCriteria, 1).Where(log => 
    log.ParticipantObjects.FirstOrDefault() != null &&
    log.ParticipantObjects[0].ParticipantObjectId == id.ToString() &&
    log.LogEventParameters[0].Value == name).OrderBy(log => 
    log.Timestamp.ToLocalTime()).ToList();