C# 0 The Complete Reference

let

 clause assigns to 

chrArray

 a reference to the array 

returned by 

str.ToCharArray( )

:

let chrArray = str.ToCharArray()

After the 

let

 clause, other clauses can make use of 

chrArray

. Furthermore, because all arrays 

in C# are convertible to 

IEnumerable

,

chrArray

 can be used as a data source for a second, 

nested

from

 clause. This is what happens in the example. It uses the nested 

from

 to enumerate 

the individual characters in the array, sorting them into ascending sequence and returning 

the result.

You can also use a 

let

 clause to hold a non-enumerable value. For example, the following 

is a more efficient way to write the query used in the 

IntoDemo

 program shown in the 

preceding section.

var webAddrs = from addr in websites

               let idx = addr.LastIndexOf(".")

               where idx != -1

               group addr by addr.Substring(idx)

                          into ws

               where ws.Count() > 2

               select ws;

In this version, the index of the last occurrence of a period is assigned to 

idx

. This value 

is then used by 

Substring( )

. This prevents the search for the period from having to be 

conducted twice.