c# - 切换 linq 语法

Question

 var folders = from r in this.bdd.Rights
               join f in this.bdd.Folders on r.RightFolderId equals f.FolderId
               join rs in this.bdd.RightSpecs on r.RightSpecId equals rs.SpecIdRight
               where r.RightUserId == userId
               where rs.SpecRead == true
               where rs.SpecWrite == true
               select f;

如何将此 linq 查询转换为其他语法？

 var folders = this.bdd.Rights.Where(r => r.....

Answer 1

阅读 C# 4 规范中的第 7.16.2 节。所有的规则都在那里。

让我们来看看吧。你有:

from r in this.bdd.Rights 
join f in this.bdd.Folders on r.RightFolderId equals f.FolderId 
join rs in this.bdd.RightSpecs on r.RightSpecId equals rs.SpecIdRight 
where r.RightUserId == userId 
where rs.SpecRead == true 
where rs.SpecWrite == true 
select f; 

规范说

A query expression with a join clause without an into followed by something other than a select clause

from x1 in e1
join x2 in e2 on k1 equals k2 
...

is translated into

from * in ( e1 ) 
. Join(e2 , x1 => k1 , x2 => k2 , ( x1 , x2 ) => new { x1 , x2 })
...

好的，我们首先将您的查询翻译成

from * in 
(this.bdd.Rights)
.Join(
    this.bdd.Folders, 
    r=>r.RightFolderId, 
    f=>f.FolderId, 
    (*, f)=> new {r, f})
join rs in this.bdd.RightSpecs on r.RightSpecId equals rs.SpecIdRight 
where r.RightUserId == userId 
where rs.SpecRead == true 
where rs.SpecWrite == true 
select f; 

现在同样的规则再次适用；我们有一个查询表达式，它有一个没有 into 的连接子句，后面跟的不是 select。这样就变成了:

from ** in 
((this.bdd.Rights)
.Join(
    this.bdd.Folders, 
    r=>r.RightFolderId, 
    f=>f.FolderId, 
    (r, f)=> new {r, f}))
.Join(
    this.bdd.RightSpecs,
    *=>r.RightSpecId,
    rs=>rs.SpecIdRight,
    (*, rs)=> new {*, rs})
where r.RightUserId == userId 
where rs.SpecRead == true 
where rs.SpecWrite == true 
select f; 

下一个适用的规则是什么？查阅规范:

A query expression with a where clause

from x in e
where f
...

is translated into

from x in ( e ) . Where ( x => f )
...

好的，所以我们应用该转换三次并得到

from ** in 
(((((this.bdd.Rights)
.Join(
    this.bdd.Folders, 
    r=>r.RightFolderId, 
    f=>f.FolderId, 
    (r, f)=> new {r, f}))
.Join(
    this.bdd.RightSpecs,
    *=>r.RightSpecId,
    rs=>rs.SpecIdRight,
    (*, rs)=> new {*, rs}))
.Where(**=>r.RightUserId == userId ))
.Where(**=>rs.SpecRead == true))
.Where(**=>rs.SpecWrite == true)
select f; 

现在呢？查阅规范:

from x in e select v

is translated into

( e ) . Select ( x => v )

所以上面的代码翻译成

((((((this.bdd.Rights)
.Join(
    this.bdd.Folders, 
    r=>r.RightFolderId, 
    f=>f.FolderId, 
    (r, f)=> new {r, f}))
.Join(
    this.bdd.RightSpecs,
    *=>r.RightSpecId,
    rs=>rs.SpecIdRight,
    (*, rs)=> new {*, rs}))
.Where(**=>r.RightUserId == userId ))
.Where(**=>rs.SpecRead == true))
.Where(**=>rs.SpecWrite == true))
.Select(**=>f);

其中 * 和 ** 是透明标识符。于是这又进一步转化为

((((((this.bdd.Rights)
.Join(
    this.bdd.Folders, 
    r=>r.RightFolderId, 
    f=>f.FolderId, 
    (r, f)=> new {r, f}))
.Join(
    this.bdd.RightSpecs,
    t1=>t1.r.RightSpecId,
    rs=>rs.SpecIdRight,
    (t1, rs)=> new {t1, rs}))
.Where(t2=>t2.t1.r.RightUserId == userId ))
.Where(t2=>t2.rs.SpecRead == true))
.Where(t2=>t2.rs.SpecWrite == true))
.Select(t2=>t2.t1.f);

我们在其中引入了很多不必要的括号。我们可以把它们拿出来，说这等同于

this.bdd.Rights
.Join(
    this.bdd.Folders, 
    r=>r.RightFolderId, 
    f=>f.FolderId, 
    (r, f)=> new {r, f})
.Join(
    this.bdd.RightSpecs,
    t1=>t1.r.RightSpecId,
    rs=>rs.SpecIdRight,
    (t1, rs)=> new {t1, rs})
.Where(t2=>t2.t1.r.RightUserId == userId )
.Where(t2=>t2.rs.SpecRead == true)
.Where(t2=>t2.rs.SpecWrite == true)
.Select(t2=>t2.t1.f);

简单易行。这只是一个简单的语法重写，除了对透明标识符的一些分析。