C# 在 foreach 循环中更改对象值？
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Changing objects value in foreach loop?
提问by
In one place i am using the list of string in that case the i am able to change the value of the string as code given below,
在一个地方，我正在使用字符串列表，在这种情况下，我可以将字符串的值更改为下面给出的代码，
foreach(string item in itemlist.ToList())
{
item=someValue; //I am able to do this
}
But for object of class i am not able to alter the members value of the object the code is as below,
但是对于类的对象，我无法更改对象的成员值，代码如下，
public class StudentDTO
{
string name;
int rollNo;
}
studentDTOList=GetDataFromDatabase();
foreach(StudentDTO student in studentDTOList.ToList())
{
studentDTO=ChangeName(studentDTO); //Not working
}
private StudentDTO ChangeName(StudentDTO studentDTO)
{
studentDTO.name=SomeName;
return studentDTO;
}
Error is : Can not assign because it's iteration variable
错误是：无法分配，因为它是迭代变量
采纳答案by Olivier Jacot-Descombes
You cannot change the iteration variable of a foreach-loop, but you can change members of the iteration variable. Therefore change the ChangeNamemethod to
您不能更改 foreach 循环的迭代变量，但可以更改迭代变量的成员。因此将ChangeName方法更改为
private void ChangeName(StudentDTO studentDTO)
{
studentDTO.name = SomeName;
}
Note that studentDTOis a reference type. Therefore there is no need to return the changed student. What the ChangeNamemethod gets, is not a copy of the student but a reference to the unique student object. The iteration variable and the studentDTOListboth reference the same student object as does the studentDTOparameter of the method.
请注意，这studentDTO是一个引用类型。因此，无需返回已更改的学生。该ChangeName方法得到的不是学生的副本，而是对唯一学生对象的引用。迭代变量和studentDTOList两者都引用相同的学生对象作为studentDTO方法的参数。
And change the loop to
并将循环更改为
foreach(StudentDTO student in studentDTOList)
{
ChangeName(student);
}
However, methods like ChangeNameare unusual. The way to go is to encapsulate the field in a property
然而，像这样ChangeName的方法是不寻常的。要走的路是将字段封装在一个属性中
private string name;
public string Name
{
get { return name; }
set { name = value; }
}
You can then change the loop to
然后您可以将循环更改为
foreach(StudentDTO student in studentDTOList)
{
student.Name = SomeName;
}
EDIT
编辑
In a comment you say that you have to change many fields. In that case it would be okay to have a method UpdateStudentthat would do all the changes; however I still would keep the properties.
在评论中，您说您必须更改许多字段。在那种情况下，可以使用一种方法UpdateStudent来完成所有更改；但是我仍然会保留这些属性。
If there is no additional logic in the properties besides passing through a value, you can replace them by the handy auto-implemented properties.
如果属性中除了传递值之外没有其他逻辑，您可以将它们替换为方便的自动实现的属性。
public string Name { get; set; }
In that case you would have to drop the field name.
在这种情况下，您将不得不删除该字段name。
回答by Jon Skeet
You're not actually changing the object that you're referring to anyway, so you can just use:
无论如何，您实际上并没有更改您所指的对象，因此您可以使用：
foreach (StudentDTO student in studentDTOList)
{
student.name = SomeName;
}
Or still call a method:
或者仍然调用一个方法：
foreach (StudentDTO student in studentDTOList)
{
ChangeStudent(student);
}
In both cases, the code doesn't change the value of the iteration variable (student) so it's okay.
在这两种情况下，代码都不会更改迭代变量 ( student)的值，所以没问题。
But your original example doesn't compile anyway - an iteration variable introduced by a foreachloop is read-only.
但是您的原始示例无论如何都无法编译 - 循环引入的迭代变量foreach是只读的。
