Linq查询在循环中添加了多个OrderBy语句

Question

我在Web服务中有一个方法,该方法具有参数,用户可以使用该参数来决定他们如何订购结果.这是一个List(Of String)带有要按其排序顺序的字段名称.

我知道我通常可以通过执行以下操作在多个列上进行排序

Dim test = Bars.OrderBy(Function(x) x.Foo) _
               .ThenBy(Function(x) x.Bar) _
               .ThenBy(Function(x) x.Test)

但是在这种情况下这不起作用,因为我无法链接ThenBy函数,因为我在循环中添加排序顺序.要使用ThenBy我需要一个IOrderedQueryable集合.这就是我希望它的工作方式

Dim sortColumns = {"Foo", "Bar", "Test"}
Dim query = From b in Bars
For each column in sortColumns
    Select Case column
        Case "Foo"
            query = query.Orderby(Function(x) x.Foo)
        Case "Bar"
            query = query.Orderby(Function(x) x.Bar)
        Case "Test"
            query = query.Orderby(Function(x) x.Test)
    End Select
Next

Dim result = query.Select(Function(x) x.x).ToList()
Return result

这当然不会起作用,因为OrderBy它将取代之前的任何订购.我能想到的唯一解决方案是首先在其他变量上排序列表,所以我已经有了一个IOrderedQueryable集合,但这似乎是错误的方法.

Dim bars As New List(Of Bar)
Dim sortColumns = {"Foo", "Bar", "Test"}
Dim query = bars.Select(Function(x) New With {.Temp = 1, .x = x}) _
                .OrderBy(Function(x) x.Temp)

For Each column In sortColumns
    Select Case column
        Case "Foo"
            query = query.ThenBy(Function(x) x.x.Foo)
        Case "Bar"
            query = query.ThenBy(Function(x) x.x.Bar)
        Case "Test"
            query = query.ThenBy(Function(x) x.x.Test)
    End Select
Next

Dim result = query.Select(Function(x) x.x).ToList()
Return result    

Answer 1

您可以编写自己的扩展方法OrderByOrThenBy来检查该值是否已经是IOrderedQueryable,ThenBy如果是,则使用OrderBy.有点臭,但不是很难做.

编辑:C#样本(未经测试):

public static class QueryableOrdering
{
    public static IOrderedQueryable<TElement> OrderByOrThenBy<TElement, TKey>
        (this IQueryable<TElement> source,
         Expression<Func<TElement, TKey>> ordering)
    {
        if (source == null)
        {
            throw new ArgumentNullException("source");
        }
        if (ordering == null)
        {
            throw new ArgumentNullException("ordering");
        }
        var ordered = source as IOrderedQueryable<TElement>;
        return ordered == null ? source.OrderBy(ordering)
                               : ordered.ThenBy(ordering);
    }
}