关于C#中Linq查询的应用

最近看了老赵关于《编程语言的发展趋势及未来方向》的博文，由于本人水平有限，还谈不能对编程语言的发展趋势发表自己的看法和见解，只是在工作之余，忙里偷闲，看看大牛们的博客，学习之余，也看到了一些关于自己平时开发中有用的东西，截取若干知识点放到这里，供自己平时学习或者工作中参考。

以下就是在这篇博文中看到的关于linq查询的一些运用：

例如，要筛选出单价大于20的产品，并对所属种类进行分组，并降序地列出每组的分类名称及产品数量。如果是用命令式的编程方式，则可能是这样的：

Dictionary<string, Grouping> groups = new Dictionary<string, Grouping>();
foreach (Product p in products)
{
if (p.UnitPrice >= 20)
{
if (!groups.ContainsKey(p.CategoryName))
{
Grouping r = new Grouping();
r.CategoryName = p.CategoryName;
r.ProductCount = 0;
groups[p.CategoryName] = r;
}
groups[p.CategoryName].ProductCount++;
}
}

List<Grouping> result = new List<Grouping>(groups.Values);
result.Sort(delegate(Grouping x, Grouping y)
{
return
x.ProductCount > y.ProductCount ? -1 :
x.ProductCount < y.ProductCount ? 1 :
0;
});

显然这些代码编写起来需要一点时间，且很难直接看出它的真实目的，换言之“What”几乎完全被“How”所代替了。这样，一个新的程序员必须花费一定时间才能理解这段代码的目的。但如果使用LINQ，代码便可以改写成：

var result = products
.Where(p => p.UnitPrice >= 20)
.GroupBy(p => p.CategoryName)
.OrderByDescending(g => g.Count())
.Select(g => new { CategoryName = g.Key, ProductCount = g.Count() });

这段代码更加关注的是“What”而不是“How”，它不会明确地给出过滤的“操作方式”，也没有涉及到创建字典这样的细节。这段代码还可以利用C# 3.0中内置的DSL，即LINQ查询语句来改写：

var result =
from p in products
where p.UnitPrice >= 20
group p by p.CategoryName into g
orderby g.Count() descending
select new { CategoryName = g.Key, ProductCount = g.Count() };

编译器会简单地将LINQ差距语句转化为前一种形式。这段代码只是表现出最终的目的，而不是明确指定做事的方式，这样便可以很容易地并行执行这段代码，如使用PINQ则几乎不需要做出任何修改。

Using the Code

1. Define the Person class that includes the ID, Name, and Age properties.

class Person
{
public Person(int id, string name, int age)
{
this.id = id;
this.name = name;
this.age = age;
}

private int id;

/// <summary>
/// Person ID
/// </summary>
public int PersonID
{
get { return this.id; }
set { this.id = value; }
}

private string name;

/// <summary>
/// Person name
/// </summary>
public string Name
{
get { return this.name; }
set { this.name = value; }
}

private int age;

/// <summary>
/// Age that ranges from 1 to 100
/// </summary>
public int Age
{
get { return this.age; }
set
{
if (value > 0 && value <= 100)
this.age = value;
else
throw new Exception("Age is out of scope [1,100]");
}
}
}

2. Build a list of Person to be used as the data source.

/////////////////////////////////////////////////////////////////////
// Build the Person list that serves as the data source.
//
List<Person> persons = new List<Person>();

persons.Add(new Person(1, "Alexander David", 20));
persons.Add(new Person(2, "Aziz Hassouneh", 18));
persons.Add(new Person(3, "Guido Pica", 20));
persons.Add(new Person(4, "Chris Preston", 19));
persons.Add(new Person(5, "Jorgen Rahgek", 20));
persons.Add(new Person(6, "Todd Rowe", 18));
persons.Add(new Person(7, "SPeter addow", 22));
persons.Add(new Person(8, "Markus Breyer", 19));
persons.Add(new Person(9, "Scott Brown", 20));

3. Use Linq to perform the query operation.

/////////////////////////////////////////////////////////////////////
// Query a person in the data source.
//
var Todd = (from p in persons
where p.Name == "Todd Rowe"
select p).First();
// [-or-]
// Use extension method + lambda expression directly
//var Todd = persons.Where(p => p.Name == "Todd Rowe").First();

Console.WriteLine("Todd Rowe's age is {0}", Todd.Age);

4. Use Linq to perform the Update operation.

/////////////////////////////////////////////////////////////////////
// Perform the Update operation on the person's age.
//
Todd.Age = 21;
Console.WriteLine("Todd Rowe's age is updated to {0}", (from p in persons
where p.Name == "Todd Rowe"
select p).First().Age);

5. Use Linq to perform the Order operation.

/////////////////////////////////////////////////////////////////////
// Sort the data in the data source.
// Order the persons by age
var query1 = from p in persons
orderby p.Age
select p;

Console.WriteLine("ID\tName\t\tAge");

foreach (Person p in query1.ToList<Person>())
{
Console.WriteLine("{0}\t{1}\t\t{2}", p.PersonID, p.Name, p.Age);
}

6. Use Linq to perform the Max, Min, Average queries.

/////////////////////////////////////////////////////////////////////
// Print the average, max, min age of the persons.
//
double avgAge = (from p in persons
select p.Age).Average();
Console.WriteLine("The average age of the persons is {0:f2}", avgAge);

double maxAge = (from p in persons
select p.Age).Max();
Console.WriteLine("The maximum age of the persons is {0}", maxAge);

double minAge = (from p in persons
select p.Age).Min();
Console.WriteLine("The minimum age of the persons is {0}", minAge);

7. Use Linq to count the Person whose age is larger than 20

/////////////////////////////////////////////////////////////////////
// Count the persons who age is larger than 20
//
var query2 = from p in persons
where p.Age > 20
select p;

int count = query2.Count();
Console.WriteLine("{0} persons are older than 20:", count);
for (int i = 0; i < count; i++)
{
Console.WriteLine(query2.ElementAt(i).Name);
}

截取此片段，只是想了解一下linq在.NET中的语法格式及简单运用，后续看到更高级运用会专门记录更新。