c# - 如何动态添加降序到orderby？

Question

我可以使用 orderby 语句动态构造 Linq 查询，如下所示:

var Query = from q in Db.TblUsers select q;

switch (Order)
{
    case "Name": Query = from q in Query orderby q.Name select q; break;
    case "DOB": Query = from q in Query orderby q.DOB select q; break;
    // ... and more cases
}

var Result = Query.ToList();

但是，如果需要降序排列(取决于用户在 UI 中的选择)，我将不得不构造另一个 switch 语句来复制所有情况，只是在 orderby 之后添加“降序”关键字。

例子:

if (ascending)
{
    switch (Order)
    {
        case "Name": Query = from q in Query orderby q.Name select q; break;
        // ....
    }
}
else
{
    switch (Order)
    {
        case "Name": Query = from q in Query orderby q.Name descending select q; break;
        // ....
    }
}

有没有办法让我动态地向查询添加降序关键字？

Answer 1

给定:

public static class OrderByEx
{
    public static IOrderedQueryable<TSource> OrderBy<TSource, TKey>(this IQueryable<TSource> source, Expression<Func<TSource, TKey>> keySelector, bool ascending)
    {
        if (ascending)
        {
            return source.OrderBy(keySelector);
        }

        return source.OrderByDescending(keySelector);
    }
}

您可以:

var Query = Db.TblUsers.AsQueryable();

switch (Order)
{
    case "Name": 
        Query = Query.OrderBy(q=>q.Name, ascending);
        break;
    case "DOB": 
        Query = Query.OrderBy(q=>q.DOB, ascending);
        break;
    // ... and more cases
}