Use of Let keyword

The LET clause allows us to store a result of an expression inside a query expression and use it on the remaining part of the query.
A let clause is useful when the query is using an expression multiple times in a query or if we simply want to create an short alias to a very long variable or property name.

For Example:

We will create a Person class which will contain two properties as FirstName and LastName,so our Person class will look like as:

public class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

Not in any event write:

List<Person> obj_persons = new List<Person>
{
    new Person { FirstName="Rajesh",LastName="Sathua" },
    new Person { FirstName="Vinod",LastName="Shinde" },
    new Person { FirstName="Preeti",LastName="Patle" },
    new Person { FirstName="Rutu",LastName="Dixit" }
};

var query = from obj in obj_persons
            let fullName = obj.FirstName + string.Empty + obj.LastName
            select new { FullName = fullName };

foreach (var person in query)
{
   Response.Write(person.FullName + "<br/>");
}

Output:
Rajesh Sathua
Vinod Shinde
Preeti Patle
Rutu Dixit

Now we can again simplify above query as:

query = obj_persons.Select(p => new { fullName = p.FirstName + string.Empty + p.LastName })
                   .Select(p => new { FullName = p.fullName });

foreach (var person in query)
{
   Response.Write(person.FullName + "<br/>");
}

Output would be the same as above.