37

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,

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,

public class StudentDTO
{
    string name;
    int rollNo;
}

studentDTOList=GetDataFromDatabase();

foreach(StudentDTO student in studentDTOList.ToList())
{
      student = ChangeName(student); //Not working 
}

private StudentDTO ChangeName(StudentDTO studentDTO)
{
     studentDTO.name = SomeName;
     return studentDTO;
}

Error is : Can not assign because it's iteration variable

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5
  • 20
    No, you're really not able to do the code in the first snippet. The iteration variable is readonly. Commented Jul 16, 2013 at 12:51
  • i am able to do because of .ToList() making the copy of list and iterating through that copy so it's working Commented Jul 16, 2013 at 12:53
  • 3
    No, you're not able to do that. Just try it. The code will not compile. If you really believe it will, please produce a short but complete program demonstrating that - you'll see it fails. Commented Jul 16, 2013 at 12:58
  • 1
    you could in loop modify the list in the ToList() clone then after the loop simply assign/set the list to equal the new ToList() modified product Commented Feb 9, 2018 at 9:57
  • 1
    just a side note, acronyms greater than 2 letters should use PascalCase. ie your class should be called StudentDto Commented Dec 11, 2018 at 3:12

3 Answers 3

Reset to default
42

You cannot change the iteration variable of a foreach-loop, but you can change members of the iteration variable. Therefore change the ChangeName method to

private void ChangeName(StudentDTO studentDTO)
{
    studentDTO.name = SomeName;
}

Note that studentDTO is a reference type. Therefore there is no need to return the changed student. What the ChangeName method gets, is not a copy of the student but a reference to the unique student object. The iteration variable and the studentDTOList both reference the same student object as does the studentDTO parameter of the method.

And change the loop to

foreach(StudentDTO student in studentDTOList)
{
    ChangeName(student);
}

However, methods like ChangeName are unusual. The way to go is to encapsulate the field in a property

private string name;
public string Name
{
    get { return name; }
    set { name = value; }
}

You can then change the loop to

foreach(StudentDTO student in studentDTOList)
{
    student.Name = SomeName;
}

EDIT

In a comment you say that you have to change many fields. In that case it would be okay to have a method UpdateStudent that would do all the changes; however I still would keep the properties.

If there is no additional logic in the properties besides passing through a value, you can replace them by the handy auto-implemented properties.

public string Name { get; set; }

In that case you would have to drop the field name.

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2 Comments

so i dont have to return anything from the function as it already getting reference type of argument
Yes. Even if you return the student, you can safely ignore the return value and not assign it to the iteration variable. Returning the student allows you to chain the method calls: student.UpdateValues().StoreChanges().PrintReport();
21

You're not actually changing the object that you're referring to anyway, so you can just use:

foreach (StudentDTO student in studentDTOList)
{
    student.name = SomeName;
}

Or still call a method:

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.

But your original example doesn't compile anyway - an iteration variable introduced by a foreach loop is read-only.

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1

Instead of Foreach, I used For loop, this way I also get the Index that is needed to be changed, Then I call a function, In which I pass the Data to be Changed as well as the index to be changed from:

  for(int i = 0; i< myList.Count; i++)
  {
    if(mylist[i] == otherData)
    {
      //call the function
      ChangeData(otherData, i);
    }
  }
  
  
  
  
  
  public void ChangeData(DataType DataToChangeInto, int i)
  {
    mylist[i] = DataToChangeInto;
  }

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