What is let keyword

Question

What is the use of let keyword. How to use the keyword

Answer 1

"Let" - is a part of a query expression.

It introduces a variable and it can be reused elsewhere in the query.

This can be make use in complex query expression to make expression simpler.

ex: 

int[] array = { 1, 3, 5, 7, 9 };

	var result = from a in array
		     let v = a
		     where v >= 5
		     select v;
"Let" key word can be used only when a new variable must be computed and reused.
Here, v - is a variable and is used twice.

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Kavithaa.N

Answer 2

It might be helpful to see an example where the let clause introduces a variable that is not just a trivial alias for another symbol, and is reused elsewhere in the query in a non-trivial way.

var query = from product in products
            let discount = product.price * 0.15m
            where discount < 50
            select product.price - discount;

It's also worth noting that with LINQ it's trickier to introduce another variable with the lambda expression.

// Not so good:
var query = products
   .Where( product => product.price * 0.15m < 50 )
   .Select( product => product.price - (product.price * 0.15m) );
// Darn! Had to write out the calculation of the discount twice.

// Better:
var query = products
   .Select( product => new { prod = product, discount = product.price * 0.15m } )
   .Where( anon => anon.discount < 50 )
   .Select( anon => anon.prod.price - anon.discount );
// Intermediate select of an anonymous object is functionally similar to the "let" clause.

Answer 3

Just to make certain there is no misscommunication:

Let is a Keyword that only works in combination with Linq Queries? I found it on the List of Keywords, under contextual Keywords:

http://msdn.microsoft.com/en-us/library/bb383976.aspx

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Let's talk about MVVM: http://social.msdn.microsoft.com/Forums/en-US/wpf/thread/b1a8bf14-4acd-4d77-9df8-bdb95b02dbe2 Please mark post as helpfull and answers respectively.

Answer 4

The Let keyword allows storing the results of a query which can be used in a subsequent query

var em = from e in emp
                     group e by new { e.Salary, e.Id }
                         into gEmp
                         let avgsal = (gEmp.Sum(t => t.Salary) / gEmp.Count())
                         where gEmp.Key.Salary == avgsal
                         select new { gEmp.Key.Salary, gEmp.Key.Id };

http://msdn.microsoft.com/en-us/library/bb383976.aspx

Answer 5

Just to make certain there is no misscommunication:

Let is a Keyword that only works in combination with Linq Queries? I found it on the List of Keywords, under contextual Keywords:

http://msdn.microsoft.com/en-us/library/bb383976.aspx

---

Let's talk about MVVM: http://social.msdn.microsoft.com/Forums/en-US/wpf/thread/b1a8bf14-4acd-4d77-9df8-bdb95b02dbe2 Please mark post as helpfull and answers respectively.

The question mark in your quoted text above is the only thing uncertain.  If you replace it with a period, you are absolutely correct.

So, at the risk of saying this needlessly, but in the spirit of giving examples:

let is a context sensitive keyword, in that it has meaning, but only in a certain context.  In this case, the context for let is that of a query clause.  let can be used as an identifier in many other contexts.  For example:

class Program
{
    class Foo
    {
        public static int let; // a field
    }
    class Bar
    {
        public static int let( int x ) // a method
        {
            return x + 1;
        }
    }
    class Baz
    {
        public static Func<int, int> fn = (let => let + 1); // a lambda variable
    }

    class let // a class name
    {
        public int x; 
    };

    static void Main( string[] args )
    {
        int let = 5; // a local variable
        let temp = new let() { x = let };
        Foo.let = Bar.let( Baz.fn(temp.x) );
    }
}

...just to give a few examples there are many other contexts in which let could be used as an identifier.

let cannot be used as an identifier in a query clause.

Here are some examples where you cannot use let as an identifier.

int[] numbers = new int[] {1, 3, 5, 7, 9};
var query = from let in numbers // Identifier expected
            select let; // Invalid expression term
var query2 = from x in numbers
             let let = x + 1 // second let gives: Identifier expected
             select x;