LINQ "let"

Sander Rossel

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

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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

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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

Nish Nishant

Same here, I never got the hang of it - never liked it to be honest, and always use C# method syntax.

Regards, Nish

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

BillWoodruff

I'm always delighted to have a chance to learn from your code ! I did add the call to 'Distinct with my first (and only) edit of the original post because I got to thinking that if you had a lot of 'if and 'else and 'and and 'is and 'or, etc., that would eliminate some duplicate calls. edit ... my head spins as I consider the ways you can use Linq to "coalesce collections:" ''SelectMany, 'Aggregate, 'Join, etc. thanks, Bill

«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

Super Lloyd

Haha, I have to say I dunno "how to replace let with extension method" so when I want to introduce a variable it's one case where I definitely use linq query syntax over chaining LINQ extension methods....

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Mark Whybird

Console.WriteLine(
string.Join(
"\n",
strings.SelectMany(sentence => sentence.Split(' '))
.Where(word => Array.IndexOf(vowels, char.ToLower(word[0])) != -1)
.Select(vowelWord => $"\"{vowelWord}\" starts with a vowel")))
;

;)

Richard Deeming

Shame there's no foreach extension method:

public static class EnumerableExtensions
{
public static void ForEach<T>(this IEnumerable<T> source, Action<T> action)
{
if (source == null) throw new ArgumentNullException(nameof(source));
if (action == null) throw new ArgumentNullException(nameof(action));

    foreach (T item in source)
    {
        action(item);
    }
}

}

Then you could do:

strings.SelectMany(sentence => sentence.Split(' '))
.Where(word => Array.IndexOf(vowels, char.ToLower(word[0])) != -1)
.Select(vowelWord => $"\"{vowelWord}\" starts with a vowel")
.ForEach(Console.WriteLine)
;

:)

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

Nicholas Marty

I think you're mistaking Linq with Linq to SQL. ;-) Depending on the use case you might even gain performance, as an Enumerable is only evaluated when it's needed. As long as you're not accessing the items (or converting it into something other than an IEnumberable with "ToList" or "ToArray") the query isn't evaluated.

Nicholas Marty

Well, you could convert the IEnumerable to a List with ToList and call the ForEach method of the List<T> class. Should pretty much equal the foreach loop (as you need to evaluate the query anyways when looping)

Sander Rossel

Nicholas Marty wrote:

I think you're mistaking Linq with Linq to SQL. ;-)

I know the difference very well. I was just assuming we were talking about LINQ to SQL/Entities/whatever data source, because if we're talking LINQ to Objects there really isn't any performance issue to talk about :)

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

James Curran

Now, if your table have appropriately assigned foreign keys, then your Profile object would have a "geekSkills" collection of Skills object, which in turn would have a Category property which would have a Name property. Reducing that to :

var categoryRanks = from s in profile.geekSkills
group s by new {Level = s.Level, Name = s.Category.Name} into g
select new SkillLevelBySkillByCategory() { 
SkillLevel = g.Key.Level, 
SkillLevelCount = g.Count(x => x.Level == g.Key.Level), 
Name = g.Key.Name };

Not having the tables (or schema) makes designing that a bit tricky, so that might be off a bit. (like, i"m pretty sure that SkillLevelCount can be just "g.Count()" but I don't know your data) Now, using LET, we can bring that down to :

var categoryRanks = from s in profile.geekSkills
let sl = new {Level = s.Level, Name = s.Category.Name}
group sl by sl into g
select new SkillLevelBySkillByCategory() {
SkillLevel = g.Key.Level,
SkillLevelCount = g.Count(x => x.Level == g.Key.Level),
Name = g.Key.Name };

Truth, James

James Curran

Which "traditional" methods? ADO with DataSets? From my experience, LINQ is trivially slower (but with added type-safety benefits to offset it). BUT, linq makes it very easy to write bad queries. Things like, reading an entire table into an array, and then linearly search through it. SO, it you compare a carefully tuned, DBA written stored procedure against a simple query written by a developer with little experience with databases, well, then, LINQ is going to lose. But, it you compare two well-crafted queries, one in LINQ and one in SQL, then you should be nearly the same (since the LINQ will generate the exact same SQL).

Truth, James