Csharp学习Linq

天神下凡 · 发表于 2024-3-19 10:37:10

Linq的学习

这里继续使用之前文章创建的学生类，首先简单介绍一下linq的使用。
Student.cs

public class Student
{
public int Id { get; set; }
public int ClassId { get; set; }
public string Name { get; set; }
public int Age { get; set; }
public string Description { get; set; }
public void Study()
{
Console.WriteLine($"{this.Id} {this.Name} 跟着老师学习 .Net开发");
}
public void StudyQt()
{
Console.WriteLine($"{this.Id} {this.Name} 跟着老师学习C++ Qt");
}
}

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初始化学生数据

public class LinqPrinciple
{
private List<Student> GetStudentsList()
{
List<Student> students = new List<Student>()
{ new Student() { Id = 1, ClassId = 1, Name = "张三", Age = 20, Description = "张三是一个好学生" },
new Student() { Id = 2, ClassId = 1, Name = "李四", Age = 21, Description = "李四是一个好学生" },
new Student() { Id = 3, ClassId = 2, Name = "王五", Age = 22, Description = "王五是一个好学生" },
new Student() { Id = 4, ClassId = 2, Name = "赵六", Age = 23, Description = "赵六是一个好学生" },
new Student() { Id = 5, ClassId = 3, Name = "孙七", Age = 24, Description = "孙七是一个好学生" },
new Student() { Id = 6, ClassId = 3, Name = "周八", Age = 25, Description = "周八是一个好学生" },
new Student() { Id = 7, ClassId = 1, Name = "绿春", Age = 30, Description = "张三是一个好学生" },
new Student() { Id = 8, ClassId = 1, Name = "麻醉", Age = 35, Description = "李四是一个好学生" },
new Student() { Id = 9, ClassId = 2, Name = "开天", Age = 26, Description = "王五是一个好学生" },
new Student() { Id = 10, ClassId = 2, Name = "匹敌", Age = 22, Description = "赵六是一个好学生" },
new Student() { Id = 11, ClassId = 3, Name = "独轮车", Age = 23, Description = "孙七是一个好学生" },
new Student() { Id = 12, ClassId = 3, Name = "火箭英", Age = 20, Description = "周八是一个好学生" }
};
return students;
}
}

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下面写几个简单的linq的语句，大概知道一下它的使用.

public void Show()
{
//获取一下ClassID为3的学生们
List<Student> students = GetStudentsList();
List<Student> studentsId=students.Where(s => s.ClassId == 3).ToList();
Console.WriteLine("********************************1*****************************");
foreach (var student in studentsId)
{
Console.WriteLine($"{student.Id} {student.Name} {student.Age} {student.Description}");
}
Console.WriteLine("********************************2*****************************");
//获取一下年纪大于24的学生们
List<Student> studentsAge = students.Where(s => s.Age > 24).ToList();
foreach (var student in studentsAge)
{
Console.WriteLine($"{student.Id} {student.Name} {student.Age} {student.Description}");
}
Console.WriteLine("********************************3*****************************");
//获取一下年纪小于23同时ClassID为2的学生们
List<Student> studentsAgeClassId = students.Where(s => s.Age < 23 && s.ClassId == 2).ToList();
foreach (var student in studentsAgeClassId)
{
Console.WriteLine($"{student.Id} {student.Name} {student.Age} {student.Description}");
}
}

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linq的原理的探究

下面尝试探索一下这个linq的本质是什么?
尝试自己实现一下这3个需求代码,如下

List<Student> students = GetStudentsList();
List<Student> studentsId=new List<Student>();
foreach (var student in students)
{
if (student.ClassId == 3)
{
studentsId.Add(student);
}
}
List<Student> studentsAge=new List<Student>();
foreach (var student in students)
{
if (student.Age > 24)
{
studentsAge.Add(student);
}
}
List<Student> studentsAgeClassId=new List<Student>();
foreach (var student in students)
{
if (student.Age < 23 && student.ClassId == 2)
{
studentsAgeClassId.Add(student);
}
}

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这上面的代码有什么问题？
大量的重复代码---最好是封装一些，把重复的代码统一封装;
这里使用一个解决方案
封装一个方法，将判断的逻辑独立到一个小方法中去.

public static bool IsOk01(Student student)
{
return student.ClassId == 3;
}
public static bool IsOk02(Student student)
{
return student.Age > 24;
}
public static bool IsOk03(Student student)
{
return student.Age < 23 && student.ClassId == 2;
}
public static List<Student> DragonWhere(List<Student> studentlist,Func<Student,bool> Func)
{
List<Student> students = new List<Student>();
foreach (var student in studentlist)
{
if (Func(student))
{
students.Add(student);
}
}
return students;
}

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尝试使用一下封装的函数

List<Student> students = GetStudentsList();
List<Student> studentId= MethodExtension.DragonWhere(students, MethodExtension.IsOk01);

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这里还可以使用扩展方法和lambda表达式进一步的优化我们的代码

public static List<Student> DragonWhere(this List<Student> studentlist,Func<Student,bool> Func)
{
List<Student> students = new List<Student>();
foreach (var student in studentlist)
{
if (Func(student))
{
students.Add(student);
}
}
return students;
}

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在调用的时候，

List<Student> students = GetStudentsList();
List<Student> studentsId = students.DragonWhere(s => s.ClassId == 3);

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这里就串联上了我们之前学到的知识，最后为了支持多种数据类型，使用上泛型，最后我们就得到了最终封装的方法.

public static List<T> DragonWhere<T>(this List<T> studentlist,Func<T,bool> Func)
{
List<T> students = new List<T>();
foreach (var student in studentlist)
{
if (Func(student))
{
students.Add(student);
}
}
return students;
}

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这里也要使用ilspy反编译一下它们的实现是如何？
看起来跟我们自己实现的Where差不多的样子.

再去看一下它的movenext函数.

这个就是一个yield return反汇编出来的状态机的代码。
linq常见的语句

public void Show()
{
List<Student> studentList = this.GetStudentList();
#region Linq 扩展方法&表达式
{
var list = studentList.Where<Student>(s => s.Age < 30); //list里面必然是符合要求的数据；
foreach (var item in list)
{
Console.WriteLine("Name={0} Age={1}", item.Name, item.Age);
}
}
{
Console.WriteLine("********************");
var list = from s in studentList
where s.Age < 30
select s; //list里面必然是符合要求的数据；
foreach (var item in list)
{
Console.WriteLine("Name={0} Age={1}", item.Name, item.Age);
}
}
#endregion
#region linq to object Show
{
Console.WriteLine("********************");
var list = studentList.Where<Student>(s => s.Age < 30)
.Select(s => new //投影：可以做一些自由组装+ new 一个匿名类，也可以new 具体类；
{
IdName = s.Id + s.Name,
ClassName = s.ClassId == 2 ? "高级班" : "其他班"
});
foreach (var item in list)
{
Console.WriteLine("Name={0} Age={1}", item.ClassName, item.IdName);
}
}
{
Console.WriteLine("********************");
var list = from s in studentList
where s.Age < 30
select new
{
IdName = s.Id + s.Name,
ClassName = s.ClassId == 2 ? "高级班" : "其他班"
};
foreach (var item in list)
{
Console.WriteLine("Name={0} Age={1}", item.ClassName, item.IdName);
}
}
{
Console.WriteLine("********************");
var list = studentList.Where<Student>(s => s.Age < 30)//条件过滤
.Select(s => new//投影
{
Id = s.Id,
ClassId = s.ClassId,
IdName = s.Id + s.Name,
ClassName = s.ClassId == 2 ? "高级班" : "其他班"
})
.OrderBy(s => s.Id)//排序升序
.ThenBy(s => s.ClassName) //多重排序，可以多个字段排序都生效
.OrderByDescending(s => s.ClassId)//倒排
.Skip(2)//跳过几条 //必须要先排序
.Take(3)//获取几条 //必须要先排序
;
foreach (var item in list)
{
Console.WriteLine($"Name={item.ClassName} Age={item.IdName}");
}
}
{//group by·
Console.WriteLine("********************");
var list = from s in studentList
where s.Age < 30
group s by s.ClassId into sg
select new
{
key = sg.Key,
maxAge = sg.Max(t => t.Age)
};
foreach (var item in list)
{
Console.WriteLine($"key={item.key} maxAge={item.maxAge}");
}
//group by new {s.ClassId,s.Age}
//group by new {A=s.ClassId>1}
}
{
Console.WriteLine("********************");
var list = studentList.GroupBy(s => s.ClassId).Select(sg => new
{
key = sg.Key,
maxAge = sg.Max(t => t.Age)
});
foreach (var item in list)
{
Console.WriteLine($"key={item.key} maxAge={item.maxAge}");
}
}
{
var list = studentList.GroupBy(s => s.ClassId);
foreach (var date in list) ///实现了IEnumerable
{
Console.WriteLine(date.Key);
foreach (var item in date)
{
Console.WriteLine(item.Age);
}
}
}
List<Class> classList = new List<Class>()
{
new Class()
{
Id=1,
ClassName="架构班"
},
new Class()
{
Id=2,
ClassName="高级班"
},
new Class()
{
Id=3,
ClassName="全栈班"
},
};
{
//Join
var list = from s in studentList
join c in classList on s.ClassId equals c.Id //只能使用equals 不能使==
select new
{
Name = s.Name,
CalssName = c.ClassName
};
foreach (var item in list)
{
Console.WriteLine($"Name={item.Name},CalssName={item.CalssName}");
}
}
{
var list = studentList.Join(classList, s => s.ClassId, c => c.Id, (s, c) => new
{
Name = s.Name,
CalssName = c.ClassName
});
foreach (var item in list)
{
Console.WriteLine($"Name={item.Name},CalssName={item.CalssName}");
}
}
{//左连接
var list = from s in studentList
join c in classList on s.ClassId equals c.Id
into scList
from sc in scList.DefaultIfEmpty()//
select new
{
Name = s.Name,
CalssName = sc == null ? "无班级" : sc.ClassName//c变sc，为空则用
};
foreach (var item in list)
{
Console.WriteLine($"Name={item.Name},CalssName={item.CalssName}");
}
Console.WriteLine(list.Count());
}
{
var list = studentList.Join(classList, s => s.ClassId, c => c.Id, (s, c) => new
{
Name = s.Name,
CalssName = c.ClassName
}).DefaultIfEmpty();//为空就没有了
foreach (var item in list)
{
Console.WriteLine($"Name={item.Name},CalssName={item.CalssName}");
}
Console.WriteLine(list.Count());
}
{
//左连接和右链接就是链接对象交换一下即可；
}
#endregion
}

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来源:https://www.cnblogs.com/wenlong-4613615/p/18073134
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Csharp学习Linq

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