Linq To Sql-使用众所周知的静态类型在运行时更改排序顺序 [英] Linq To Sql - Changing Sort Order At Run-Time with well known static typing

问题描述

这不是关于如何动态排序(基于用户提供的任意字段)吗?"的另一个问题.

This is not another question about 'How Can I Sort Dynamically (based on an arbitrary user provided field)?'

问题是-当我提前知道潜在的排序方式时，如何更改排序顺序? (从而避免通常与真正的动态排序相关联的反射/自定义表达式构建.)

The question is -- how can I change sort order when I know the potential sorts in advance? (And thus avoid reflection / custom Expression building typically associated with truly dynamic sorting.)

以一个较大查询的此子查询(此示例为简称)为例:

Take for instance this subquery (shortened for this example) of a larger query:

(from solutionIds in context.csExtendedQAIncident_Docs
    where solutionIds.tiRecordStatus == 1
    && (from solutionProductAssocation in context.csProductDocs
           where solutionProductAssocation.iSiteId == Settings.Current.WebUtility().Onyx.SiteId
           && (from allowedProduct in context.KB_User_Allowed_Products
                  where allowedProduct.UserId == userId
                  select allowedProduct.ModelCode
               ).Contains(solutionProductAssocation.chModelCd)
           select solutionProductAssocation.chIdNo).Distinct().Contains(solutionIds.chIdNo)
).OrderByDescending(s => s.dtUpdateDate)
.Select(s => s.chIdNo)
.Take(count ?? Settings.Current.WCFServices().Output.HomePage.MaxRows)

OrderByDescending部分按预期工作.

The OrderByDescending portion works as I would expect.

现在-我想像以下那样将其排除在外:

Now -- I want to factor that out like the following:

Expression<Func<csExtendedQAIncident_Doc, IComparable>> ordering =  (s) => s.dtUpdateDate;
if (viewType == HomepageViewType.MostViewed)
   ordering = (s) => s.vchUserField8;
else if (viewType == HomepageViewType.MostEffective)
   ordering = (s) => s.vchUserField4;

然后使用:

OrderByDescending(ordering)

这确实可以编译，但是在运行时会炸毁.

This does compile, but blows up at run-time.

Unsupported overload used for query operator 'OrderByDescending'.  

这当然来自System.Data.Linq.SqlClient.QueryConverter的深层内容-特别是VisitSequenceOperatorCall.反映该代码表明，必须满足以下条件才能正确评估OrderByDescending. "mc"是传递给方法的MethodCallExpression.

This of course comes from deep in the bowels of System.Data.Linq.SqlClient.QueryConverter -- in particular VisitSequenceOperatorCall. Reflectoring that code reveals that the following conditions must be met for OrderByDescending to properly evaluate. 'mc' is the MethodCallExpression passed into the method.

if (((mc.Arguments.Count != 2) || !this.IsLambda(mc.Arguments[1]))
    || (this.GetLambda(mc.Arguments[1]).Parameters.Count != 1))
{
   break;
}

从本质上讲，MethodCallExpression必须具有2个参数，其中第二个必须是具有单个参数(可能是排序字段)的Expressions.LambdaExpression.如果该代码崩溃，则会抛出我所收到的异常.

So essentially that MethodCallExpression has to have 2 arguments, the second of which has to be a Expressions.LambdaExpression with a single parameter (presumably the sort field). If that code breaks out, the exception that I got is thrown.

很明显，我没有正确构造表达式.在这里不做进一步探讨，有谁知道如何正确构造排序表达式?

So clearly I have not constructed the expression correctly. Without digging in any further here, does anyone know how to correctly construct the sorting Expression?

推荐答案

我认为您的代码不支持的部分是使用IComparable作为您的排序表达式的常规返回类型.如果考虑普通使用OrderByDescending，则编译器生成的lambda表达式的返回类型为您要排序的属性的类型:例如，字符串属性的Expression<Func<csExtendedQAIncident_doc, string>>.

I think the unsupported part of your code is the use of IComparable as a general return type for your ordering expression. If you consider the plain use of OrderByDescending, the compiler-generated lambda expression has a return type of the type of the property that you're ordering by: for example, an Expression<Func<csExtendedQAIncident_doc, string>> for a string property.

一个可能的答案，尽管我不确定它是否适合您的情况，但首先创建一个无序查询:

One possible answer, although I'm not sure whether it works in your case, is to first create an unordered query:

IQueryable<Foo> unorderedQuery = from f in db.Foo select f;

然后，根据排序:

IOrderedQueryable<Foo> orderedQuery = unorderedQuery
    .OrderBy(f => f.DefaultSortKey);

if (sortBy == SortByName)
    orderedQuery = unorderedQuery.OrderBy(f => f.Name);
else if (sortBy == SortByDate)
    orderedQuery = unorderedQuery.OrderBy(f => f.Date);
// etc.

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