Human LINQ

Last night I gave a talk about C# 3 and LINQ, organised by Iterative Training and NxtGenUG. I attempted to cover all the features of C# 3 and the basics of LINQ in about an hour and a half or so. It’s quite a brutal challenge, and obviously I wasn’t able to go into much detail about anything. It went down reasonably well, but I can’t help feeling there’s a lot of room for improvement. That said, there was one part of the talk which really did go well, and made the appropriate points effectively.

I had demonstrated the following LINQ query in code:

using System;
using System.Linq;

class Test
    static void Main(string[] args)
        var words = new[] {“the”, “quick”, “brown”, “fox”,
                “jumped”, “over”, “the”, “lazy”, “dog”};
        var query = from x in words
                    where x.Length > 3
                    where x[0] != ‘q’
                    select x.ToUpper();
        foreach (string word in query)

I know it would have been easy to combine the two “where” clauses, but separating them helped with the second part of the demonstration.

In the pizza break, I had prepared sheets of paper – some with the words on (“the”, “quick” etc), and some with clauses (“from x in words”, “where x.Length > 3” etc). I asked for 5 volunteers, who I arranged in a line, facing the rest of the audience. I stood at “audience left” with the sheets of words, gave the next person the “from” clause, then the first “where” clause etc. The person at the far end didn’t have a sheet of paper, but they were acting as the foreach loop.

I suspect you can see what’s coming – we ran possibly the slowest LINQ query in the world. However, we did it reasonably accurately: when the next piece of information was needed, a person would turn to their neighbour and request it; the request would pass up the line until it got to me, whereupon I’d hand over a sheet of paper with a word on. If a “where” clause filtered out a word, they just dropped the piece of paper. When a word reached the far end, the guy shouted it out.

With this, I was able to illustrate:

  • Deferred execution (nothing starts happening until the foreach loop executes)
  • Streaming (only a single word was ever on the move at once)

Next we added an “orderby” clause in just before the end. Sure enough, we then see buffering in action – the guy representing the ordering can’t return any data to the “select” clause until he’s actually got all the data.

Finally, we removed the “orderby” clause again, but added a call to Count(). We didn’t have time to go into a lot of detail, but I think people understood why that led to immediate execution rather than the deferred execution we had earlier.

I suspect I’m not the first person to do something like this, but I’m still really pleased with it. If you’re ever talking about LINQ and people’s eyes are glazing over, it’s a fun little demo. It wasn’t perfect though; there are things I’d change:

  • Put the upper case version of the word on the back of the paper. We had to imagine the result of the projection.
  • Having two “where” clauses is useful for the first demo, but slows things down after that.
  • Possibly use fewer words – it takes quite a while, and having been through it three times, the audience may grow impatient.
  • Explain deferred execution more in terms of the result type – it’ll make it easier to contrast with immediate execution

Overall, it was a really fun night. I did a little interview with Dave McMahon afterwards, which should go up on the NxtGenUG site at some point. I suspect I was talking rather too quickly for the whole time, but we’ll see how it pans out.

4 thoughts on “Human LINQ”

  1. It is a bit out there, but I like it! :-)

    Got me thinking about using a similar exercise to demonstrate why certain things in PLINQ are the way they are.


  2. Peter – unfortunately I won’t be at the summit. I had planned to be, but leaving Holly with 3 kids for a week “just for fun” is a bit much.

    As it happens, I may be in the US over the summit for a different reason, but one which would have clashed even if I’d registered for the summit.

    Maybe next time (which is what I said last time too…)

    It would certainly have been great to meet up with the C# team – and plug the book, of course.



  3. I just read a book called “Made to Stick” about how you need to make ideas “concrete” for them to stick. You example illustrates this.


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