在LINQ中使用表达式树到实体框架核心

Question

我想从数据库中按我选择的顺序来获取数据，例如，我想让网站上注册的所有用户按以下顺序排序：名称，名称升序，注册日期或评论数。

我不想重复所有的select语句，因为仅按语句排序

那是我的代码，但不适用于我：

Expression<Func<AppUser, Object>> OrderByExpression = null;

switch (userOrder)
{
    case UserOrder.RegistDate:
        OrderByExpression = a => a.RegistrationDate;
        break;
    case UserOrder.A_Z:
        OrderByExpression = a => a.UserName;
        break;
    case UserOrder.Z_A:
        OrderByExpression = a => a.UserName descending;
        break;
    case UserOrder.MostComment:
        OrderByExpression = a => a.Comments.Count;
        break;
}

那就是Select语句：

IEnumerable<AppUser> AllUsers = 
    (from a in context.Users
     orderby OrderByExpression.Compile()(a)
     select new AppUser
     {
         UserName = a.UserName,
         Id = a.Id,
         Email = a.Email,
         EmailConfirmed = a.EmailConfirmed,
         RegistrationDate = a.RegistrationDate

     }).ToList();

它工作正常，但当我想按评论订购时出现问题。 计数 ＆用户名降序 ，问题出在（ 计数 ＆ 降序 ）

Answer 1

有一个更简单的解决方案，只需使用LINQ的流畅接口即可：

var query = context.Users.AsQueryable();

switch (userOrder)
{
    case UserOrder.RegistDate:
        query = query.Orderby(a => a.RegistrationDate);
        break;
    case UserOrder.A_Z:
        query = query.Orderby(a => a.UserName);
        break;
    case UserOrder.Z_A:
        query = query.OrderbyDescending(a => a.UserName);
        break;
    case UserOrder.MostComment:
        query = query.Orderby(a => a.Comments.Count);
        break;
}

IEnumerable<AppUser> results = query
    .Select(a => new AppUser
    {
        UserName = a.UserName,
        Id = a.Id,
        Email = a.Email,
        EmailConfirmed = a.EmailConfirmed,
        RegistrationDate = a.RegistrationDate
    })
    .ToList();