LINQ选择日期是昨天。

DateTime yesterday = DateTime.Today.AddDays(-1);
YesterdaysRegistrations = db.tblForumAuthors.Where(c => c.Join_date == yesterday).Count();

AddDays

DateTime yesterday = DateTime.Today.AddDays(-1).Date;

DateTime yesterday = DateTime.Today.AddDays(-1).Date;
DateTime yesterdayEnd = DateTime.Today.Date.AddSeconds(-1);
db.tblForumAuthors.Where(c => c.Join_date >= yesterday && c.Join_date < yesterdayEnd)

DateTime today = DateTime.Today; // read once, avoid \"odd\" errors once in a blue moon
DateTime yesterday = today.AddDays(-1);
YesterdaysRegistrations = db.tblForumAuthors.Where(
    c => c.Join_date >= yesterday
      && c.Join_date < today).Count();