2

我希望能够在 linq 表达式中重构OrderBy子句。

这是一个重构where子句 的例子

前：

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() });

后：

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;
}

我想知道除了我想重构Orderby表达式之外，是否可以进行相同类型的事情。

先感谢您

4

3 回答 3

3

假设您想要实际获取一个字符串，例如“LastName”、“FirstName”等，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做到这一点，但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。

于 2008-12-17T16:10:01.587 回答
1

谢谢乔恩，

这个答案让我现在走上了正确的道路......但是，我需要在我的订单中保留ThenBy子句。所以虽然下面的解决方案不是动态的，但它仍然保留了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);
    }
}
于 2008-12-17T20:14:48.330 回答
0

我同意 Jon 在可能的情况下使用 lambdas 来避免拼写错误等。但是，如果你真的不能这样做（无论出于何种原因），我过去看过完全动态OrderBy的。有关在 LINQ-to-SQL 上测试的示例，请参见此处（但理论上 EF 应该没问题）。

于 2008-12-17T19:46:55.797 回答