c# - 按 lambda 表达式动态排序

Question

我正在我的应用程序中进行排序，如下所示。

public IQueryable<Users> SelectAll(string sSortExpression, string sSortOrder)
{
    if (sSortOrder == "asc")
    {
        switch (sSortExpression)
        {
            case "FirstName":
                return UsersRepository.Entities.OrderBy(x => x.FirstName);
            case "LastName":
                return UsersRepository.Entities.OrderBy(x => x.LastName);
            default:
                return UsersRepository.Entities.OrderBy(x => x.Id);
        }
    }
    else
    {
        switch (sSortExpression)
        {
            case "FirstName":
                return UsersRepository.Entities.OrderByDescending(x => x.FirstName);
            case "LastName":
                return UsersRepository.Entities.OrderByDescending(x => x.LastName);
            default:
                return UsersRepository.Entities.OrderByDescending(x => x.UserName);
        }
    }
}

现在没问题了，但我必须对 Users 表中的所有字段(大约 30 个字段)进行排序。那么方法会很大

我试过像这样使用反射

public IQueryable<Users> SelectAll(string sSortExpression, string sSortOrder)
{
    var _property = UsersRepository.GetType().GetProperties().Where(a => a.Name == sSortExpression);
    if (sSortOrder == "asc")
    {
        return UsersRepository.Entities.OrderBy(x => _property);
    }
    else
    {
        return UsersRepository.Entities.OrderByDescending(x => _property);
    }
}

但是失败了。

有没有更好的方法来做到这一点？提前致谢

Answer 1

最简单的方法:更改您的方法以接受 Expression 而不是 string:

public IQueryable<Users> SelectAll<TProp>(Expression<Func<Users, TProp>> selector, string sSortOrder)
{
    if (sSortOrder == "asc")
    {
        return UsersRepository.Entities.OrderBy(selector);
    }
    else
    {
        return UsersRepository.Entities.OrderByDescending(selector);
    }
}

你可以这样调用它:

SelectAll(x => x.LastName, "asc");

或者如果你真的需要它是字符串，你必须使用 System.Linq.Expressions.Expression 类方法生成表达式树:

public IQueryable<Users> SelectAll<TProp>(string sSortExpression, string sSortOrder)
{
    var param = Expression.Parameter(typeof(Users));
    var propExpression = Expression.Lambda<Func<Users, TProp>>(Expression.Property(param, sSortExpression), param);

    if (sSortOrder == "asc")
    {
        return UsersRepository.Entities.OrderBy(propExpression);
    }
    else
    {
        return UsersRepository.Entities.OrderByDescending(propExpression);
    }
}

但它需要在 SelectAll 调用时指定泛型类型参数:

var results = SelectAll<int>("Id", "asc");