重构 OrderBy 表达式

Question

我希望能够重构 linq 表达式中的 OrderBy 子句。

以下是重构 where 子句

before:

results = ctx.ActiveUsers
    .Where(u => u.CompanyID != 1 &&
           (u.LastName.ToLower().Contains(searchString)
           || u.Email.ToLower().Contains(searchString)
           || u.Company.Name.ToLower().Contains(searchString)))
    .OrderBy(u =>  u.LastName ).ThenBy(u => u.FirstName)
    .Select(u => new Employee {
      ID = u.ID
      , FirstName = u.FirstName
      , LastName = u.LastName
      , Email = u.Email
      , CompanyName = u.Company.Name
      , CompanyID = u.CompanyID.ToString() });

---

after:

results = ctx.ActiveUsers
    .Where(Employee.GetExpression(searchString))
    .OrderBy(u =>  u.LastName ).ThenBy(u => u.FirstName)
    .Select(u => new Employee {
      ID = u.ID
      , FirstName = u.FirstName
      , LastName = u.LastName
      , Email = u.Email
      , CompanyName = u.Company.Name
      , CompanyID = u.CompanyID.ToString() });

---

private static Expression<Func<User, bool>> GetExpression(string searchString)
{ 
    Expression<Func<User, bool>> p = (u => u.CompanyID != 1 &&
                       (u.LastName.ToLower().Contains(searchString)
                       || u.Email.ToLower().Contains(searchString)
                       || u.Company.Name.ToLower().Contains(searchString)));
    return p;
}

我想知道是否可以实现相同类型的事情，除非我想重构 Orderby 表达式。

先感谢您

Answer 1

假设您实际上想要获取“LastName”、“FirstName”等字符串，我会执行以下操作：

var unordered = ctx.ActiveUsers
                   .Where(Employee.GetExpression(searchString))
                   .OrderBy(ordering)
                   .Select(u => new Employee {
                       ID = u.ID,
                       FirstName = u.FirstName,
                       LastName = u.LastName,
                       Email = u.Email,
                       CompanyName = u.Company.Name,
                       CompanyID = u.CompanyID.ToString() });

并添加一个新的 OrderBy 扩展方法：

public static class UserQueryableExtensions
{
    public static IOrderedQueryable<User> OrderBy(this IQueryable<User> source,
                                                  string ordering)
    {
        switch (ordering)
        {
            case "LastName":
                return source.OrderBy(x => x.LastName);
            case "FirstName":
                return source.OrderBy(x => x.FirstName);
            case "Email":
                return source.OrderBy(x => x.Email);
            case "Company":
                return source.OrderBy(x => x.Company);
            default:
                throw new ArgumentException("Unknown ordering");
        }
    }
}

您当然可以使用反射来执行此操作，但是除非您有一组重要的属性（或者您想对不同的实体类型使用相同的例程），否则 switch 语句会更容易。

Answer 2

谢谢乔恩，

这个答案让我现在走上了正确的道路......但是，
我需要在订单中保留 ThenBy 子句。 因此，虽然下面的解决方案不是动态的，但它仍然保留 ThenBy

var unordered = ctx.ActiveUsers
                   .Where(Employee.GetExpression(searchString))
                   .MyOrder()
                   .Select(u => new Employee {
                       ID = u.ID,
                       FirstName = u.FirstName,
                       LastName = u.LastName,
                       Email = u.Email,
                       CompanyName = u.Company.Name,
                       CompanyID = u.CompanyID.ToString() });

---

public static class UserQueryableExtensions
{
    public static IOrderedQueryable<User> MyOrder(this IQueryable<User> source )
    { 
        return source.OrderBy(x => x.LastName).ThenBy(x => x.FirstName).ThenBy(x => x.Email);
    }
}

Answer 3

我同意 Jon 的观点，即尽可能使用 lambda 以避免拼写错误等。但是，如果您确实无法做到这一点（无论出于何种原因），我已经在过去的。 请参阅此处在 LINQ-to-SQL 上测试的示例（但理论上使用 EF 应该没问题）。