LINQ的＆QUOT;无法转换前pression ......到SQL，不能把它作为一个地方的前pression＆QUOT。 [英] Linq "Could not translate expression... into SQL and could not treat it as a local expression."

问题描述

我开始了<一个href=\"http://stackoverflow.com/questions/1262736/only-one-ex$p$pssion-can-be-specified-in-the-select-list-when-the-subquery-is-not\">this问题，这我有点回答<一个href=\"http://stackoverflow.com/questions/1262736/only-one-ex$p$pssion-can-be-specified-in-the-select-list-when-the-subquery-is-not/1262794#1262794\">there,现在我在这里要求更根本的问题。我已经简化了查询到这一点：

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 是结合了编号和扩展字段成整齐的格式化字符串。而这里的导致错误：

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.

如果我更改参考上述从 FormattedNumber 来只是普通的编号，一切工作正常。

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

不过，我想格式化数字在我的名单上完美呈现。你有什么建议的最巧妙的，这样做的干净的方式？

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

您可以使用 AsEnumerable 的实体，但是这将迫使它带回所有列（即使不使用）;或许反而是这样的：

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 一些方法，它采用了两个合并它们，从您的其他（财产）code presumably再使用。

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

使用LINQ到SQL，另一个选择是揭露一个UDF上，做数据库内的数据格式化上下文;稍有不同的例子：

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)
          };

（将做的工作在数据库;不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)

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