在LINQ to Entity Framework Core中使用表达式树

Question

我希望通过我选择的东西从数据库中获取数据，例如，我希望获得在网站中注册的所有用户：名称，名称升序，注册日期或注释数量。

我不想重复所有select语句，因为只有order by语句

这是我的代码，但它不能与我合作：

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，但问题，当我想通过Comments.Count和UserName降序排序，问题是（计数和降序）

Answer 1

有一个更简单的解决方案，只需使用LINQ的流畅界面：

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();