Code equivalent to the 'let' keyword in chained LINQ extension method calls

问题

Using the C# compilers query comprehension features, you can write code like:

var names = new string[] { "Dog", "Cat", "Giraffe", "Monkey", "Tortoise" };
var result =
    from animalName in names
    let nameLength = animalName.Length
    where nameLength > 3
    orderby nameLength
    select animalName;

In the query expression above, the let keyword allows a value to be passed forward to the where and orderby operations without duplicate calls to animalName.Length.

What is the equivalent set of LINQ extension method calls that achieves what the "let" keyword does here?

回答1:

Let doesn't have its own operation; it piggy-backs off of Select. You can see this if you use "reflector" to pull apart an existing dll.

it will be something like:

var result = names
        .Select(animalName => new { nameLength = animalName.Length, animalName})
        .Where(x=>x.nameLength > 3)
        .OrderBy(x=>x.nameLength)
        .Select(x=>x.animalName);

回答2:

There's a good article here

Essentially let creates an anonymous tuple. It's equivalent to:

var result = names.Select(
  animal => new { animal = animal, nameLength = animal.Length })
.Where(x => x.nameLength > 3)
.OrderBy(y => y.nameLength)
.Select(z => z.animal);

回答3:

There is also a .Let extension method in System.Interactive, but its purpose is to introduce a lambda expression to be evaluated 'in-line' in a fluent expression. For instance, consider (in LinqPad, say) the following expression that creates new random numbers every time it's executed:

var seq = EnumerableEx.Generate(
    new Random(),
    _ => true,
    _ => _,
    x => x.Next());

To see that new random samples show up every time, consider the following

seq.Zip(seq, Tuple.Create).Take(3).Dump();

which produces pairs in which the left and right are different. To produce pairs in which the left and right are always the same, do something like the following:

seq.Take(3).ToList().Let(xs => xs.Zip(xs, Tuple.Create)).Dump();

If we could invoke lambda expressions directly, we might write

(xs => xs.Zip(xs, Tuple.Create))(seq.Take(3).ToList()).Dump();

But we can't invoke lambda expressions as if they were methods.

回答4:

about Code equivalent to the 'let' keyword in chained LINQ extension method calls

above comment is no more valid

var x = new List<int> { 2, 3, 4, 5, 6 }.AsQueryable();
(from val in x
let val1 = val
let val2 = val + 1
where val2 > val1
select val
).Dump();

produces

System.Collections.Generic.List`1[System.Int32]
.Select(
  val =>
     new
     {
         val = val,
         val1 = val
     }
)
.Select(
  temp0 =>
     new
     {
         temp0 = temp0,
         val2 = (temp0.val + 1)
     }
)
.Where(temp1 => (temp1.val2 > temp1.temp0.val1))
.Select(temp1 => temp1.temp0.val)

so multiple let are optimized now

来源：https://stackoverflow.com/questions/1092687/code-equivalent-to-the-let-keyword-in-chained-linq-extension-method-calls

标签

linq

extension-methods

linq-to-objects