LINQ "let"

Marc Clifton

Learned something new today. I was googling for CRC algorithms and came across this nifty site[^] and started perusing it more generally, then realized I had no idea about let clauses in query expressions![^] Geez, I've been using LINQ for a while now, and didn't know about that. :doh: Marc

Imperative to Functional Programming Succinctly Contributors Wanted for Higher Order Programming Project! Learning to code with python is like learning to swim with those little arm floaties. It gives you undeserved confidence and will eventually drown you. - DangerBunny

Jacquers

Cool! I didn't know about it either. Now to find a use for it, just because :)

Pete OHanlon

It's a useful tool in your arsenal. So how have you been solving this in the past? Multiple queries chained together? Just wait until you start using "into[^]".

This space for rent

Rob Philpott

I didn't know that and will probably forget quite soon too. Do people use that sort of syntax still? I never see it, and don't know what its called. My inclination would be something hideous like this:

var a = strings.SelectMany(x => x.Split(' ')).Select(x => x.ToLower()).Where(x => x[0] == 'a' || x[0] == 'e' || x[0] == 'i' || x[0] == 'o' || x[0] == 'u');

...which I would call Linq, but may not be, or might be just some perverse form of it.

Regards, Rob Philpott.

Richard Deeming

That's the method chaining syntax. The other version is the query syntax. Query Syntax and Method Syntax in LINQ (C#)[^] I tend to prefer the method chaining syntax for most queries, but for certain queries, the query syntax looks cleaner.

"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

OriginalGriff

I can't get my head round the Linq syntax, so I always use method chaining. I think it's the way the query is backwards (just like SQL) where method chaining fits C# syntax better (in my mind anyway).

Bad command or file name. Bad, bad command! Sit! Stay! Staaaay...

Kenneth Haugland

Ah, you might like the LINQ to objects book[^] then, it is brilliant. (At least I think it is :) ) Tons of useful tips in it, and a very nice read as well.

Rob Philpott

Agreed. So my next question would be: Is there anything you can do in one syntax that you can't in the other? Personally, if it takes more than a moment to figure out what it's trying to do, I revert to pre-2007 methods.

Regards, Rob Philpott.

Richard Deeming

Query syntax is rewritten by the compiler to the same series of method calls as the method chaining syntax, so there's nothing you can do in QS that you can't do in MCS. There are a few things that look a bit neater in QS - let being a prime example - but there's always a way to write the same query in MCS. There are quite a few extension methods that don't have an equivalent query keyword, so there are things you can do in MCS that you can't do in QS. LINQPad[^] is probably the best tool to compare the two syntaxes.

"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

9082365

You're not missing anything. I trialled Linq functions extensively a couple of years back and they were always slower (sometimes markedly so) than the traditional methods they 'replace'. It all looks very fancy and sophisticated but it's totally inefficient.

I am not a number. I am a ... no, wait!

Sander Rossel

Yeah, let is great. Sometimes I convert a query from method syntax to query syntax just so I can use let :)

Read my (free) ebook Object-Oriented Programming in C# Succinctly. Visit my blog at Sander's bits - Writing the code you need. Or read my articles here on CodeProject.

Simplicity is prerequisite for reliability. — Edsger W. Dijkstra

Regards, Sander

Sander Rossel

You're missing out on a lot! LINQ can be a bit slower, but it's awesome for many use cases. When you really need the milliseconds go for regular ADO.NET, but how often do you really need that? My experience with LINQ is not that it's slow to use, but that people suddenly forget that their LINQ expression becomes a SQL query and start writing the most horrible, non-indexed queries, now THAT is a performance killer. What I really like about LINQ is that you can create your own extension methods and use those to create queries that read like regular sentences, or just a lot better than SQL in general (I mean, who remembers why all those WHERE clauses are there?) :)

Read my (free) ebook Object-Oriented Programming in C# Succinctly. Visit my blog at Sander's bits - Writing the code you need. Or read my articles here on CodeProject.

Simplicity is prerequisite for reliability. — Edsger W. Dijkstra

Regards, Sander

Rob Philpott

Splendid, thanks for the info.

Regards, Rob Philpott.

Marc Clifton

Pete O'Hanlon wrote:

So how have you been solving this in the past? Multiple queries chained together?

My Linq tends to be rather simple. :)

Pete O'Hanlon wrote:

Just wait until you start using "into[^]".

Though for reporting, yup, been there, done that:

var categoryRanks = (from gs in geekSkills
where (gs.ProfileId == profile.Id)
join s in skills on gs.SkillId equals s.Id
select new { Level = gs.Level, CategoryId = s.CategoryId } into gss
join c in categories on gss.CategoryId equals c.Id
select new { Level = gss.Level, Name = c.Name } into gssc
group gssc by new { gssc.Name, gssc.Level } into g
select new SkillLevelBySkillByCategory() {
SkillLevel = g.Key.Level,
SkillLevelCount = g.Count(x => x.Level == g.Key.Level),
Name = g.Key.Name });

Marc

Imperative to Functional Programming Succinctly Contributors Wanted for Higher Order Programming Project! Learning to code with python is like learning to swim with those little arm floaties. It gives you undeserved confidence and will eventually drown you. - DangerBunny

Marc Clifton

OriginalGriff wrote:

I can't get my head round the Linq syntax, so I always use method chaining.

I tend to use both, depending on what I'm doing. For example, this:

var categoryRanks = (from gs in geekSkills
where (gs.ProfileId == profile.Id)
join s in skills on gs.SkillId equals s.Id
select new { Level = gs.Level, CategoryId = s.CategoryId } into gss
join c in categories on gss.CategoryId equals c.Id
select new { Level = gss.Level, Name = c.Name } into gssc
group gssc by new { gssc.Name, gssc.Level } into g
select new SkillLevelBySkillByCategory() {
SkillLevel = g.Key.Level,
SkillLevelCount = g.Count(x => x.Level == g.Key.Level),
Name = g.Key.Name });

seems more readable to me than method chaining, but I also do things like this:

T record = mappedRecords[typeof(T)].Cast().Where(r => r.Row == row).Single();

because here, it flows better. Marc Marc

Imperative to Functional Programming Succinctly Contributors Wanted for Higher Order Programming Project! Learning to code with python is like learning to swim with those little arm floaties. It gives you undeserved confidence and will eventually drown you. - DangerBunny

Marc Clifton

Rob Philpott wrote:

Is there anything you can do in one syntax that you can't in the other?

Possibly Cast<T> For example:

T record = mappedRecords[typeof(T)].Cast().Where(r => r.Row == row).Single();

But I'm not sure, I haven't seen any examples using query syntax. I suppose the point though is, you should cast before you query. :rolleyes: Marc

Imperative to Functional Programming Succinctly Contributors Wanted for Higher Order Programming Project! Learning to code with python is like learning to swim with those little arm floaties. It gives you undeserved confidence and will eventually drown you. - DangerBunny

BillWoodruff

Hi Marc, I first became aware of the 'Let and 'Into Linq operators through this 2011 CP article (which I down-voted for "lack of original content"): [^]. I have never used them :) because I have never invested the energy to learn to use the "fuller" query syntax (my bad). Your comment makes me wonder what I am missing (other than motivation). cheers, Bill p.s. the Microsoft example of 'Let you cite imho goes to a lot trouble do this:

string[] strings =
{
"A penny saved is a penny earned.",
"The early bird catches the worm.",
"The pen is mightier than the sword."
};

string vowels = "aeiou";

List<string> vowelstartwords = String.Join(" ", strings)
.Split(' ')
.Distinct()
.Where(word => vowels.Contains(Char.ToLower(word[0])))
.ToList();

That example, taken as a programming challenge, interests me: it leaves me wondering if it could be significantly improved in terms of memory use and execution time, and if the Linq code using 'Let would, in fact, improve those usage parameters.

«There is a spectrum, from "clearly desirable behaviour," to "possibly dodgy behavior that still makes some sense," to "clearly undesirable behavior." We try to make the latter into warnings or, better, errors. But stuff that is in the middle category you don’t want to restrict unless there is a clear way to work around it.» Eric Lippert, May 14, 2008

Richard Deeming

The Cast<T> part isn't a problem for query syntax - you just declare the type on the variable in the from clause:

from T mappedRecord in mappedRecord[typeof(T)]
...

But there's no query syntax keyword for Single, so you still have to call that as a method:

T record = (from T mappedRecord in mappedRecords[typeof(T)]
where mappedRecord.Row == row
select mappedRecord).Single();

I think the method chaining syntax is much cleaner - especially if you use the overload of Single to eliminate the Where call:

T record = mappedRecords[typeof(T)].Cast<T>().Single(r => r.Row == row);

"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

Richard Deeming

I'd be inclined to avoid joining the strings just to split them again. I'd also be inclined to use an array of char, rather than searching a string - although I doubt it would make much difference. You've also added a Distinct and a ToList which weren't in the original example. :)

char[] vowels = { 'a', 'e', 'i', 'o', 'u' };

IEnumerable<string> vowelStartWords = strings
.SelectMany(sentence => sentence.Split(' '))
.Where(word => Array.IndexOf(vowels, char.ToLower(word[0])) != -1)
;

"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

Nish Nishant

Marc, you probably know this by now, but let is just syntactic sugar for the Select method.

Regards, Nish

Website: www.voidnish.com Blog: voidnish.wordpress.com