let子句用於在LINQ表達式中存儲子表達式的計算結果。let子句創建一個範圍變量來存儲結果，變量被創建後，不能修改或把其他表達式的結果重新賦值給它。
此範圍變量可以再後續的LINQ子句中使用。

實例1

 1 int[] numbers = new[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
 2 
 3 //傳統下的子查詢做法
 4 //var query = from num in numbers
 5 //            select num * (from n in numbers
 6 //                          where n % 2 == 0
 
 7 //                          select n).Count();
 8 
 9 //使用LET關鍵字的做法
10 var query = from num in numbers
11          let evenNumbers = from n in numbers
12                     where n % 2 == 0
13                          select n
14         select num * evenNumbers.Count();
15 
16 foreach (var item in query)
 
17 {
18     Console.WriteLine(item);
19 } 

運行結果：0 5 10 15 20 25 30 35 40 45

實例2

 1 string[] strings = { "A penny saved is a penny earned.", "The aaxly sdj", "the pa is no" };
 2 
 3 var query = from sentence in strings
 4             let words = sentence.Split(‘ ‘)//用空格分割成數組                
 5             from
 
 word in words
 6             let w = word.ToLower()//把每個字母小寫        
 7             where w[0] == ‘a‘ || w[0] == ‘e‘
 8             select word;
 9 
10 foreach (var s in query)
11 {
12     Console.WriteLine(s);
13 }

運行結果：

實例3

 1 List<Person> persons = new List<Person>
 2 {
 3     new Person { ID = "0001", Name = "張三" ,Gender = "男" , Age = 18},
 4     new Person { ID = "0002", Name = "李四" ,Gender = "女" , Age = 17},
 5     new Person { ID = "0003", Name = "王五" ,Gender = "女" , Age = 19},
 6     new Person { ID = "0004", Name = "趙六" ,Gender = "男", Age = 20}
 7 };
 8 var query = from p in persons
 9             let friendlyName = p.Gender == "男" ? "Mr" + p.Name : "Ms" + p.Name
10             select new
11             {
12                 UserID = p.ID,
13                 FriendName = friendlyName
14             };
15 foreach (var item in query)
16 {
17     Console.WriteLine("No:{0},Friendly Name:{1}", item.UserID, item.FriendName);
18 }

運行結果：

實例4

 1 List<Person> persons = new List<Person>
 2 {
 3     new Person { ID = "0001", Name = "張三" ,Gender = "男" , Age = 18},
 4     new Person { ID = "0002", Name = "李四" ,Gender = "女" , Age = 17},
 5     new Person { ID = "0003", Name = "王五" ,Gender = "女" , Age = 19},
 6     new Person { ID = "0004", Name = "趙六" ,Gender = "男", Age = 20}
 7 };
 8 
 9 var query = from p in persons
10             let gender = p.Gender
11             let age = p.Age
12             where age > 18 && gender == "女"
13             select p;
14 foreach (var item in query)
15 {
16     Console.WriteLine("姓名: " + item.Name + "  年齡:" + item.Age);
17 }

運行結果：

LINQ之let關鍵字