IF null 或空集 0 LINQ

Question

如果没有记录为空或为空，我尝试让此表显示值 0，我正在计算相等的字符串，我的字符串是“sm_red”。

MVC LinQ .NET Framework 4.5

List<string> trae = new List<string>();
            trae = db.SM_App.Where(x => x.sm_canal.ToString() == "EVERYTHING").Select(x => x.sm_categoria).Distinct().ToList();

var query = (from a in trae
             join b in db.SM_App on a equals b.sm_categoria
             where b.sm_canal == "EVERYGHINT" && b.sm_fecha > sDate && b.sm_fecha < fDate
             group b by b.sm_categoria into g
             select new
             {
             Categoria = g.Key,
             Facebook = g.Count(x => x.sm_red == "Facebook"),
             Twitter = g.Count(x => x.sm_red == "Twitter"),
             Instagram = g.Count(x => x.sm_red == "Instagram") 
             }).Distinct().ToList();

2019 年 13 月 8 日更新

我试试这个，但是返回“Count = 0”，并且在视图中什么都不返回

var query = (from a in trae
          join b in db.SM_App on a equals b.sm_categoria into grp
          from c in grp.DefaultIfEmpty()
          where c.sm_canal == "EVERY" && c.sm_fecha > sDate && c.sm_fecha < fDate       
          group c by c.sm_categoria into g
          select new 
          {
            Categoria = g.Key,
            Facebook = g.Count(x => x.sm_red == "Facebook").ToString() == "" ? Convert.ToInt32(g.Key) : 0,
            Twitter = g.Count(x => x.sm_red == "Twitter").ToString() == "" ? Convert.ToInt32(g.Key) : 0,
            Instagram = g.Count(x => x.sm_red == "Instagram").ToString() == "" ? Convert.ToInt32(g.Key) : 0
           }).Distinct().ToList();

Answer 1

如果您的整个查询返回零记录，那么您的结果中将有零记录。如果你想要一个包含0s 的记录，你必须使用DefaultIfEmpty()：

var query = (from a in trae
             join b in db.SM_App on a equals b.sm_categoria
             where b.sm_canal == "EVERYGHINT" && b.sm_fecha > sDate && b.sm_fecha < fDate
             group b by b.sm_categoria into g
             select new {
                 Categoria = g.Key,
                 Facebook = g.Count(x => x.sm_red == "Facebook"),
                 Twitter = g.Count(x => x.sm_red == "Twitter"),
                 Instagram = g.Count(x => x.sm_red == "Instagram") 
             }).Distinct()
               .DefaultIfEmpty(new {
                   Categoria = default(string),
                   Facebook = 0,
                   Twitter = 0,
                   Instagram = 0 
               })
               .ToList();

我不知道在这种情况下您期望什么Categoria 或它的类型，所以我只假设字符串和default(string) 是null。