LINQ let Keyword

 

The new keyword “let” which is used inside LINQ queries to create temporarily variables.

check this out

var list = new List<int>() { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

var query = from i in list

            let j = i + 2

            let k = j * j

            select new { i, j, k };

As you can see the let keyword was used to create two new temp variables (j, k) which their values were calculated inside the query.

Also with let query clauses, you can introduce a variable into scope and use it in the subsequent query clauses. Similar to local variables in a method body, this gives you a way to avoid evaluating a common expression multiple times by storing it in a variable.

For instance, inexperienced Linq developer quickly codes up a query that looks like this:

var nameList = new List<string>

                   {

                       "Matt",

                       "Adam",

                       "John",

                       "Peter",

                       "Owen",

                       "Steve",

                       "Richard",

                       "Chris"

                   };

var vowels = new List<string> {"A", "E", "I", "O", "U"};

var names = (from p in nameList                                                  

            where

            (vowels.Any(v => p.ToUpper().StartsWith(v)) 

            || vowels.Any(v => p.ToUpper().EndsWith(v))) &&

            (p.Length == 4 

            || p.Length == 5)

            select p).ToList();

 

Using “let”, we can define four intermediate variables that hold our vowels and booleans for our tests, then in the where clause we just check our boolean values:

var names = (from p in nameList

            let vowels = new List<string> { "A", "E", "I", "O", "U" }

            let startsWithVowel = vowels.Any(v => p.ToUpper().StartsWith(v))

            let endsWithVowel = vowels.Any(v => p.ToUpper().EndsWith(v))

            let fourCharactersLong = p.Length == 4

            let fiveCharactersLong = p.Length == 5

            where

            (startsWithVowel || endsWithVowel) &&

            (fourCharactersLong || fiveCharactersLong)

            select p).ToList();

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