Can I have an incrementing count variable in LINQ?

linq linqpad
Mike Pateras picture Mike Pateras · Feb 5, 2010 · Viewed 29.5k times · Source

I want to do something like this:

from a in stuff
let counter = 0
select new { count = counter++, a.Name };

But I get a error telling me that counter is read only. Is there a way to do something similar to this, without declaring a variable outside of the query?

Basically, I just want to show a count/index column in LINQPad (which is awesome, BTW), which means I can't declare counter ahead of time.

Answer

Jon Skeet picture Jon Skeet · Feb 5, 2010

Rather than using side-effects, use the overload of Select which takes an index:

stuff.Select((value, index) => new { index, value.Name });

You could do it using side-effects, but not in the way you tried:

int counter = 0;
var query = from a in stuff
            select new { count = counter++, a.Name };

I would strongly advise against this though.

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