C#进行数据筛选（二）

这里介绍LINQ+Lambda表达式进行数据筛选的方式

这里是第一种方式，还是使用了if条件语句去判断，根据选择的条件去筛选出我所需要的数据

 public GxAnaly SelectDay(string ID, DateTime Day)
        {
            try
            {
                DBEntities db = new DBEntities();
                var temp = (from pt in db.Tb_Analysis
                           where pt.ID == Sn
                           select pt).ToList();
               // return temp.Any() ? LoadFromModel(temp.First()) : null;
                if (temp.Count == 0)
                {
                    return null;
                }
                else
                {
                    List<Analy> gxDetail = new List<Analy>();
                    List<Analy> Detail = new List<Analy>();
                    temp.ForEach(e => gxDetail.Add(LoadFromModel(e)));
                    foreach (Analy Detail in gxDetail)
                    {
                        if (Detail.OtTime.Day == Day.Day&&Detail.OtTime.Month==Day.Month&&Detail.OtTime.Year==Day.Year)
                        {
                            Detail.Add(Detail);
                        }
                    }
                    if (Detail.Count == 0)
                    {
                        return null;
                    }
                    else
                    {
                        return Detail.FirstOrDefault();
                    }
                }
            }
            catch (Exception ex)
            {
                return null;
                Logger.Error(ex.Message + ex.StackTrace);
            }
        }

第二种方式，直接用Lambda自带的筛选属性

using(var db=new MyDbContext())

{

var s= db.Students.ToList().First(s=>s.ID=10086);

}

//这里写的是进行全表查询，不建议使用

//根据相应的条件进行查询，不会去查询全表，特别在数据量庞大的时候，建议加个判断using(var db=new MyDbContext())

{

var list =db.Students.AsQueryable();

if(********){list=list.Where(s=>s.ID=1200);}

if(******){list=list.Where(...)}

}

下面展示一个多个Lambda表达式进行表操作的方法

    public static class PredicateBuilder
    {

        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<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)
        {
            // build parameter map (from parameters of second to parameters of first)
            var map = first.Parameters.Select((f, i) => new { f, s = second.Parameters[i] }).ToDictionary(p => p.s, p => p.f);

            // replace parameters in the second lambda expression with parameters from the first
            var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);

            // apply composition of lambda expression bodies to parameters from the first expression
            return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
        }

        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
        {
            return first.Compose(second, Expression.And);
        }

完美的动态拼接Lambda表达式如下：

using (var db = new MyDbContext())
            {
                var predicate = PredicateBuilder.True<Student>();
                predicate=predicate.And(s => s.ID > 1200);
                predicate=predicate.Or(s => s.ID < 1000);
                var result = db.Students.Where(predicate).ToList();
            }

下面是一种多Lambda表达式操作的一个范例，这里部分引用来自https://blog.csdn.net/leewhoee/article/details/8968023，自己懒得去敲了，顺手就借来用了

using (var db = new SHTrackerDbContext())
            {

                var predicate = PredicateBuilder.True<Course>();
                settings = DecorateSettings(settings);

                Expression<Func<Course, bool>> checkCourse = c => db.Students.Any(s => s.CourseID == c.ID);
                if (!string.IsNullOrEmpty(settings.Quater_Year))
                {

                    checkCourse =
                        c => db.Students.Any(s => s.CourseID == c.ID && db.Student2CBOs.Any(
                            s2c => s2c.StudentID == s.ID && s2c.Quater_Year.Equals(settings.Quater_Year)));
                }
                if (settings.QuaterYearArray != null)
                {
                    checkCourse =
                        c => db.Students.Any(s => s.CourseID == c.ID && db.Student2CBOs.Any(
                            s2c =>
                            s2c.StudentID == s.ID && settings.QuaterYearArray.Any(qy => qy.Equals(s2c.Quater_Year))));
                }

                if (!string.IsNullOrEmpty(settings.DPU_ID))
                {
                    checkCourse =
                        checkCourse.And(
                            c => db.Students.Any(s => s.CourseID == c.ID && s.DPU_ID.Equals(settings.DPU_ID)));
                }
                predicate = predicate.And(checkCourse);

                if (settings.IsCheckInstructorName)
                {
                    predicate = predicate.And(c => c.InstructorName.Equals(settings.InstructorName));
                }
                if (!string.IsNullOrEmpty(settings.Term))
                {
                    predicate = predicate.And(c => c.TermDescription.Equals(settings.Term));
                }
                if (settings.TermArray != null)
                {
                    predicate = predicate.And(c => settings.TermArray.Any(t => t.Equals(c.TermDescription)));
                }
                if (settings.CourseType != CourseType.All)
                {
                    predicate = predicate.And(c => c.Type == (int) settings.CourseType);
                }
                var cc =
                    new CourseCollection(
                        db.Courses.AsNoTracking()
                          .Where(predicate)
                          .OrderByDescending(m => m.ID)
                          .Skip((pageIndex - 1)*pageSize)
                          .Take(pageSize)
                          .ToList(),
                        db.Courses.AsNoTracking().Where(predicate).Count())
                        {
                            PageIndex = pageIndex,
                            PageSize = pageSize,
                            Settings = DecorateSettings(settings)
                        };

                return cc;
            }

原文地址：https://www.cnblogs.com/yinxuejunfeng/p/9095878.html