一起养成写作习惯！这是我参与「掘金日新计划 · 4 月更文挑战」的第4天，点击查看活动详情。
前言
今天我们来讲一讲C#的linq语法，以及特性，这都是我们学习.netcore的基础，我们一起来看一看吧！
linq(where)
linq to object ： 数组、list集合等 --内存里的数据
linq to sql : 查询数据库 --数据库中的数据
linq to XML : 查询XML文件
1.自定义linq
public class linqshow{
public void show{
List<Students> list3 = StudenList.AntWhere(x => x.Age<20).ToList();
}
}
public static class LinqExtend {
public static IEnumerable<TSource> AntWhere<TSource>(this IEnumerable<TSource> resource, Func<TSource,bool> func){
List<TSource> list2 = new List<TSource>();
foreach(var item in resource){
if(func.Invoke(item)){
list2.Add(item)
}
}
}
}
2.官方的Linq（扩展方法方式）
//var list4 = StudenList.Where(x=>x.Age<20);
List<Students> list4 = StudenList.Where(x=>x.Age<20).ToList();
3.表达式方式的
List<Students> list5 = (from s in StudentList
where s.Age<20
select s).ToList();
Linq(Select)
select就是映射到另一个对象
public void show(){
//扩展方法用法
var query = StudentList.Select(s => new StudentModel{
Id=s.Id,
Name=s.Name
})
//表达式用法
//var query2 = from s in StudentList
// select new StudentModel{
// Id=s.Id,
// Name=s.Name
// }
}
Linq(Join)
多表连接查询
public List<JobviewModel> GetJobs()
{
//var jobs = from j in _context.Jobs
// join cp in _context.Companys on j.CompanyId equals cp.Id
// join ct in _context.Cities on j.WorkPlace equals ct.Id
// select new JobviewModel
// {
// Id = j.Id,
// JobName = j.JobName,
// JobPay = j.JobPay,
// Welfare = j.Welfare,
// WorkExperince = j.WorkExperince,
// WorkPlace = j.WorkPlace,
// WorkArea = j.WorkArea,
// PublishTime = j.PublishTime,
// PositionInfo = j.PositionInfo,
// Company = j.Company,
// City = j.City
// };
//return jobs.ToList();
var jobs= _context.Jobs.Join(_context.Companys, j => j.CompanyId, c => c.Id, (j, c) => new JobviewModel
{
Id = j.Id,
JobName = j.JobName,
JobPay = j.JobPay,
Welfare = j.Welfare,
WorkExperince = j.WorkExperince,
WorkPlace = j.WorkPlace,
WorkArea = j.WorkArea,
PublishTime = j.PublishTime,
PositionInfo = j.PositionInfo,
Company = c,
City = j.City
}).Join(_context.Cities, jv => jv.WorkPlace, c => c.Id, (jv, c) => new JobviewModel
{
Id = jv.Id,
JobName = jv.JobName,
JobPay = jv.JobPay,
Welfare = jv.Welfare,
WorkExperince = jv.WorkExperince,
WorkPlace = jv.WorkPlace,
WorkArea = jv.WorkArea,
PublishTime = jv.PublishTime,
PositionInfo = jv.PositionInfo,
Company = jv.Company,
City = c
}).ToList();
return jobs;
}
Linq的其他方法
var query = StudentList
// .Skip(6) 跳过
// .Take(6) 取
// .OederBy(s=>s.Name) 排序
// .OederByDescending(s=>s.Id) 倒序
.where(s => s.Name.Contains("A")) 模糊查询
IQueryable类型
实现了IEumerable接口
数据库的数据用这种类型接收
特性
应用场景：框架上、类上面、方法上面、属性上面、字段上面、参数上面。。。
特性的本质：类，继承自（Attribute类）
特性简单的定义、查找、使用
（定义--》使用--》通过反射查找特性--》调用）
//pragram.cs
Student student = new Student();
string tableName = CustomAttribute.GetTableNameAttribute(student)
//TableAttribute.cs
//特性配置 [AttributeUsage(AttributeTarget.Class,AllowMutiple=true,Inherited = true)]
//特性定义
public class TableAttribute : Attribute{
public string TableName{get;set;}
public TableAttribue(){
}
public TableAttribue(string tableName){
TableNametableName
}
}
//CustomeAttribute.cs
(这里通过反射得到表名)
public CustomeAttribute{
public static string GetCustomAttribute<T>(T mode) where T:class{
Type type = typeof(T);
if(type.IsDefined(typeof(TableAttribute),true)){
var attribute = type.GetCustomeAttributes(typeof(TableAttribute),true)
return ((TableAttribute)attribute[0]).TableName;
}
else{
return type.Name
}
}
}
常用的特性编写
//student.cs
public class Student{
[key] //这个属性是主键
public int Id{get;set;}
[StringLength(maxinumLength:50,MiniumLength:6)] //字符串长度
public string Name{get;set;}
[EmailAdress] //识别邮箱地址
public string Email{get;set;}
[Required] //属性不能为空
public int Age{get;set;}
[Display(Name="电话号码")] //显示字段别名
public int PhoneNumber{get;set;}
}
定义一个抽象类 ,其他定义的特性类继承它（方便代码的重用）：
//CommonValidateAttribute.cs
abstract class CommonValidateAttribute:Attribute
{
public abstract bool IsValidate(object dataValue);
}
后面就各种特性类继承（实现抽象类）：
//KeyAttribute.cs
public class KeyAttribute:CommonValidateAttribute{
punblic bool IsPrimaryKey{get;set} = true;
public override bool IsValidate(object dataValue){
return IsPrimaryKey;
}
}
//RequiredAttribute.cs
public class RequiredAttribute:CommonValidateAttribute{
public override bool IsValidate(object dataValue){
return dataValue！=null&&dataValue.ToString().length!=0 ;
}
}
....
反射查找：
//CustomAttribute.cs
public static bool Validate<T>(T model)
{
//获取所有的属性和特性】
PropertyInfo propertyInfos = model.GetType().GetPropeties();
//遍历属性和读取特性
foreach(var property in propertyInfos)
{
if(property.IsDefined(typeof(CommonValidateAttribute),true))
{
var attributes = property.GetCustomAttribute(typeof(CommonValidateAttribute),true)
foreach(item in attributes)
{
CommonValidateAttribute attribute= item as CommonValidateAttribute;
if(!attribute.IsValidate(property.GetValue(model)))
{
return false
}
}
}
}
return true
}
调用：
//pragram.cs
Student stu = new Student(){
Id=1,
Name="aaa",
Age=18,
Email="123456789@qq.com",
PhoneNumber="123456789"
};
var result = CustomAttribute.IsValidate(stu);
