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。