In LINQ, what does the select new keyword combination do?

I haven't found much documenta开发者_如何学运维tion on this.

Thanks

By using select new, you can use the data or objects in the set you are working with to create new objects, either typed anonymously or normally.

1. Using select new to return new anonymously typed objects:

var records = from person in people
              select new { person.name, person.id };

This uses the new keyword to create a set of anonymously typed objects. A new object type is simply created by the compiler, with two properties in it. If you look at the object type in the debugger, you'll see that it has a crazy-looking, auto-generated type name. But you can also use the new keyword just like you're used to outside of linq:

2. Using select new to construct "regularly" typed objects:

var records = from person in people
              select new CreditRecord(person.creditLimit, person.name);

This example uses the new keyword just like you're used to - to construct some known type of object via one of its constructors.

Select is called a transformation (or projection) operator. It allows you to put the data in the set that you are working with through a transformation function, to give you a new object on the other side. In the examples above, we're simply "transforming" the person object into some other type by choosing only specific properties of the person object, and doing something with them. So the combination of select new ... is really just specifying the new operation as the transformation function of the select statement. That might make more sense with a counter example to the above two:

3. Using select without new, with no transformation

Of course you do not need to use select and new together. Take this example:

var someJohns =  from person in people
                 where person.name == "John"
                 select person;

This gives you back the original object type that was in the set you were working with - no transformation, and no new object creation.

4. Using select without new, with a transformation

And finally, a transformation without the new keyword:

var personFoods = from person in people
                  select person.GetFavoriteFoods();

which gives you back some new type of object, generated from the transformation function and not by directly using the new keyword to construct it.

It's the select keyword followed by a new expression, probably creating an anonymous type.

In other words, it's a combination of the following two unrelated expressions:

select

from a in whatever select 2

Anonymous types

new { Prop1 = 3, Prop2 = DateTime.Now }

It's not really a combination, new just creates an anonymous type to return the value. Otherwise you would have to work with Dictionaries/Arrays etc (like in PHP) which is always ugly.

var q = 
    from staff in theWhiteHouseStaff
    select new { Name = staff.Name } ;