GOTO是退出双Foreach的唯一途径吗？

Question

可能重复:
在vb.net中嵌套/退出

是GOTO从双重的ForEach退出的唯一途径？

For Each city in cities
  For Each person in city.People
    If IsOK(person) GOTO Found_
  Next person
Next city

Found_: 
' ...

问题是VB.NET,但也想知道C#...

Answer 1

把它放在单独的功能中

  Private Function FindPerson(cities As List(of City)) As Person
     For Each city in cities
        For Each person in city.People
           If IsOK(person) Return person
        Next person
     Next city
     Return Nothing
  End Function

和...

  Private Function ContainsPerson(cities As List(of City)) As Bool  
     For Each city in cities
        For Each person in city.People
           If IsOK(person) Return True
        Next person
     Next city
     Return False
  End Function

编辑:修复VB语法

Answer 2

正如海因齐在这个问题中的回答:

不幸的是,没有exit two levels of for声明,但有一些解决方法可以避免Goto,这被认为是不好的做法:

• 假外块

  Do
      For Each item In itemList
          For Each item1 In itemList1
              If item1.Text = "bla bla bla" Then
                  Exit Do
              End If
          Next
      Next
  Loop While False
  

  要么

  Try
      For Each item In itemlist
          For Each item1 In itemlist1
              If item1 = "bla bla bla" Then
                  Exit Try
              End If
          Next
      Next
  Finally
  End Try
  

• 单独的功能:将循环放在一个单独的功能中,可以退出return.但是,这可能需要您传递大量参数,具体取决于您在循环中使用的局部变量数量.另一种方法是将块放入多行lambda中,因为这会在局部变量上创建一个闭包.

• 布尔变量:这可能会使您的代码的可读性降低,具体取决于您拥有多少层嵌套循环:

  Dim done = False
  
  For Each item In itemList
      For Each item1 In itemList1
          If item1.Text = "bla bla bla" Then
              done = True
              Exit For
          End If
      Next
      If done Then Exit For
  Next
  

Answer 3

为什么不使用LINQ？

C#:

// Or use SingleOrDefault(...) if there can ever only be one.
var person = Cities.SelectMany(city => city.People)
                   .FirstOrDefault(person => IsOK(person));

if (person != null)
{
    ...
}

VB.Net(我最好的尝试,我不是在其中详细说明):

// Or use SingleOrDefault(...) if there can ever only be one.
Dim person = Cities.SelectMany(Function(city) city.People)
                   .FirstOrDefault(Function(person) IsOK(person));

If person Not Nothing Then
    ...
End If

如果你要做的就是查看是否有IsOK(person),那么请使用Any(...)扩展方法:

C#:

var isOK = Cities.SelectMany(city => city.People)
                 .Any(person => IsOK(person));

VB.Net(我最好的尝试,我不是在其中详细说明):

Dim isOK = Cities.SelectMany(Function(city) city.People)
                 .Any(Function(person) IsOK(person));