Code First Entity Framework和Lambda表达式First/Where可能会组成的陷阱

环境：.NET Framework 4.0, Code First ENtity Framework 4.x

Lambda表达式First和Where在取一条数据时可以互换。但是当关联多表并使用First时就会出现错误，下面是错误之一：

Unable to create a constant value of type 'ICStars2_0.Model.Article2Category'. Only primitive types or enumeration types are supported in this context.

报错代码：

using (var db = new DbContext())
{
return
db.Categories.First(c => db.Article2Categories.Any(
a2c =>
a2c.CategoryID == c.ID &&
db.Articles.Any(a => a.UrlTitle.Equals(urlTitle) && a.ID == a2c.ArticleID)));
}

解决方案：

using (var db = new DbContext())
{
return
db.Categories.Where(
c =>
db.Article2Categories.Any(
a2c =>
a2c.CategoryID == c.ID &&
db.Articles.Any(a => a.UrlTitle.Equals(urlTitle) && a.ID == a2c.ArticleID))).First();
}

对应SQL语句：

select * from category where exists(

select * from article2category where category.ID=CategoryID and exists(

select * from article where ID=article2category.ArticleID and UrlTitle='urlTitle'

)

)

当使用Resharper重构时，就会把正确的WHERE子句改成FIRST子句，进而报错。 有时需要慎用Resharper, 它会重构出冗余代码又可能会重构出错误。