Linq“无法将表达式...转换为 SQL 并且无法将其视为本地表达式."

问题描述

我开始使用 这个问题，我回答了那里，现在我在这里问更基本的问题.我已将查询简化为:

I started out with this question, which I sort of answered there, and now I'm asking the more fundamental question here. I've simplified the query down to this:

var q = from ent in LinqUtils.GetTable<Entity>()
        from tel in ent.Telephones.DefaultIfEmpty()
        select new {
          Name = ent.FormattedName,
          Tel = tel != null ? tel.FormattedNumber : "" // this is what causes the error
        };

tel.FormattedNumber 是一个将 Number 和 Extension 字段组合成一个格式整齐的字符串的属性.这是导致的错误:

tel.FormattedNumber is a property that combines the Number and Extension fields into a neatly formatted string. And here's the error that results:

System.InvalidOperationException: Could not translate expression 'Table(Entity).SelectMany(ent => ent.Telephones.DefaultIfEmpty(), (ent, tel) => new <>f__AnonymousType0`2(Name = ent.FormattedName, Tel = IIF((tel != null), tel.FormattedNumber, "")))' into SQL and could not treat it as a local expression.

如果我将上面的引用从 FormattedNumber 更改为简单的 Number，则一切正常.

If I change the reference above from FormattedNumber to just plain Number, everything works fine.

但我确实希望格式化的数字能很好地显示在我的列表中.您推荐的最简洁、最干净的方法是什么?

But I do want the formatted number to display nicely in my list. What do you recommend as the neatest, cleanest way of doing so?

推荐答案

您可以在实体上使用 AsEnumerable，但这会强制它带回所有列(即使未使用)；也许是这样的:

You could use AsEnumerable on the entity, but that would force it to bring back all the columns (even if not used); perhaps instead something like:

var q1 = from ent in LinqUtils.GetTable<Entity>()
         from tel in ent.Telephones.DefaultIfEmpty()
         select new {
           Name = ent.FormattedName,
           Number = (tel == null ? null : ent.Number),
           Extension = (tel == null ? null : ent.Extension)
         };

var q2 = from row in q1.AsEnumerable()
         select new {
             row.Name,
             FormattedNumber = FormatNumber(row.Number, row.Extension)
         };

其中 FormatNumber 是某种将两者合并并合并它们的方法，大概是从您的其他(属性)代码中重用的.

where FormatNumber is some method that takes the two and merges them, presumably re-used from your other (property) code.

对于 LINQ-to-SQL，另一种选择是在数据上下文中公开一个 UDF，用于在数据库内部进行格式化；一个稍微不同的例子:

With LINQ-to-SQL, another option is to expose a UDF on the data-context that does the formatting inside the database; a slightly different example:

var qry = from cust in ctx.Customers // and tel
          select new {
              cust.Name,
              FormattedNumber = ctx.FormatNumber(tel.Number, tel.Extension)
          };

(它将在数据库中完成工作；无论这是否是一个好主意；-p)

(which will do the work at the database; whether or not that is a good idea ;-p)