[Lambda表达式]简单应用-HQY 一个和谐有爱的空间

13

2024
08
19:10:28

[Lambda表达式]简单应用

lambda表达式多条件查询

并且，与或者：&&与||

clist = clist.Where(r => r.Price > 50 && r.Price < 60 || r.Price < 30).ToList();

包含：contains

List<car> clist= new cardata().SelectAll().Where(r => r.Name.Contains(TextBox1.Text)).ToList();

等于：==

clist = clist.Where(r => r.Price==40).ToList();

clist = clist.Where(r => r.Price.Equals(40)).ToList();

以。。开始、以。。结束

List<users> ulist = con.users.Where(r => r.username.StartsWith("s")).ToList();
            List<users> ulsit = con.users.Where(r => r.username.EndsWith("t")).ToList();

最大值、最小值、平均值、求和

int s = con.users.Max(r=>r.ids);
            int t = con.users.Min(r=>r.ids);
            double h = con.users.Average(r => r.ids);
            int d = con.users.Sum(r => r.ids);

求交集

            List<users> ulist1 = new List<users>();
            List<users> ulist2 = new List<users>();
            List<users> ulist3 = new List<users>();
            List<users> ulist = ulist1.Intersect(ulist2).Intersect(ulist3).ToList();

顺序、倒序

            List<users> ulist = con.users.OrderBy(r => r.ids).ToList();
            List<users> ulist1 = con.users.OrderByDescending(r => r.ids).ToList();

//******* 对集合按Name属于进行分组GroupBy查询 ********
            //结果中包括的字段：
            //1、分组的关键字：Name = g.Key
            //2、每个分组的数量：count = g.Count()
            //3、每个分组的年龄总和：ageC = g.Sum(item => item.Age)
            //4、每个分组的收入总和：moneyC = g.Sum(item => item.Money)

            //写法1：Lambda 表达式写法（推荐）
            var ls = persons1.GroupBy(a => a.Name).Select(g => (new { name = g.Key, count = g.Count(), ageC = g.Sum(item => item.Age), moneyC = g.Sum(item => item.Money) }));
            //写法2：类SQL语言写法最终编译器会把它转化为lamda表达式
            var ls2 = from ps in persons1
                     group ps by ps.Name
                         into g
                         select new { name = g.Key, count = g.Count(), ageC = g.Sum(item => item.Age), moneyC = g.Sum(item => item.Money) };

            dataGridView1.DataSource = ls.ToList();
           //dataGridView1.DataSource = ls2.ToList();
        }

List.Where(new Func<Class, bool>(Where));

        private bool Where(Class c)
        {
            //组织表达式，返回bool
        }

Expression<Func<XXX, bool>> expression = t => true;
expression = expression.And(t => t.AAA == 1);
expression = expression.And(t => t.BBB > 1);
var ds = products.AsQueryable().Where(expression).ToList();

public static class PredicateExtensions
    {
        public static Expression<Func<T, bool>> True<T>()
        {
            return f => true;
        }

        public static Expression<Func<T, bool>> False<T>()
        {
            return f => false;
        }

        public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expression1, Expression<Func<T, bool>> expression2)
        {
            var invokedExpression = Expression.Invoke(expression2, expression1.Parameters.Cast<Expression>());
            return Expression.Lambda<Func<T, bool>>(Expression.Or(expression1.Body, invokedExpression), expression1.Parameters);
        }

        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expression1, Expression<Func<T, bool>> expression2)
        {
            var invokedExpression = Expression.Invoke(expression2, expression1.Parameters.Cast<Expression>());
            return Expression.Lambda<Func<T, bool>>(Expression.And(expression1.Body, invokedExpression), expression1.Parameters);
        }
    }