How do I manipulate an object's properties after it has been added to a List in C#

Question

Say I have a class like this:

class public Person
{
    public string firstName;
    public string lastName;
    public string address;
    public string city;
    public string state;
    public string zip;

    public Person(string firstName, string lastName)
    {
        this.firstName = firstName;
        this.lastName = lastName;
    }
}

And let's further say I create a List of type Person like this:

List<Person> pList = new List<Person>;
pList.Add(new Person("Joe", "Smith");

Now, I want to set the address, city, state, and zip for Joe Smith, but I have already added the object to the list. So, how do I set these member variables, after the object has been added to the list?

Thank you.

You really shouldn't be exposing the fields publicly; it would be more appropriate to use properties. Thanks to auto implemented properties it's very little additional effort. — Servy, Feb 01 '13 at 16:15
I appreciate the advice. Can you please elaborate further with an example? — user717236, Feb 01 '13 at 16:16
@user717236 http://stackoverflow.com/questions/40730/how-do-you-give-a-c-sharp-auto-property-a-default-value — Dom, Feb 01 '13 at 16:18
@user717236: I edited my answer to show a bit the use of such automatic properties. — Jesse Emond, Feb 01 '13 at 16:21

score 4 · Answer 1 · answered Feb 01 '13 at 16:14

4

You get the item back out of the list and then set it:

pList[0].address = "123 Main St.";

answered Feb 01 '13 at 16:14

Servy

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score 3 · Answer 2 · answered Feb 01 '13 at 16:15

3

You can keep a reference to your object around. Try adding like this:

List<Person> pList = new List<Person>;
Person p = new Person("Joe", "Smith");
pList.Add(p);
p.address = "Test";

Alternatively you can access it directly through the list.

pList[0].address = "Test";

answered Feb 01 '13 at 16:15

Cashley

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5
16

Well, no, actually, you don't need to keep a reference around. You *can*, but you don't *need* to. – Servy Feb 01 '13 at 16:15

Jesse Emond · Accepted Answer · 2013-02-01T16:20:51.707

You can get the first item of the list like so:

Person p = pList[0]; or Person p = pList.First();

Then you can modify it as you wish:

p.firstName = "Jesse";

Also, I would recommend using automatic properties:

class public Person
{
    public string firstName { get; set; }
    public string lastName { get; set; }
    public string address { get; set; }
    public string city { get; set; }
    public string state { get; set; }
    public string zip { get; set; }

    public Person(string firstName, string lastName)
    {
        this.firstName = firstName;
        this.lastName = lastName;
    }
}

You'll get the same result, but the day that you'll want to verify the input or change the way that you set items, it will be much simpler:

class public Person
{
    private const int ZIP_CODE_LENGTH = 6;
    public string firstName { get; set; }
    public string lastName { get; set; }
    public string address { get; set; }
    public string city { get; set; }
    public string state { get; set; }
    private string zip_ = null;
    public string zip 
    { 
        get { return zip_; } 
        set
        {
            if (value.Length != ZIP_CODE_LENGTH ) throw new Exception("Invalid zip code.");
            zip_ = value;
        }
    }

    public Person(string firstName, string lastName)
    {
        this.firstName = firstName;
        this.lastName = lastName;
    }
}

Quite possibly not the best decision to just crash when you set a property here, but you get the general idea of being able to quickly change how an object is set, without having to call a SetZipCode(...); function everywhere. Here is all the magic of encapsulation an OOP.

score 1 · Answer 4 · answered Feb 01 '13 at 16:17

You can access the item through it's index. If you want to find the last item added then you can use the length - 1 of your list:

List<Person> pList = new List<Person>;
// add a bunch of other items....
// ....
pList.Add(new Person("Joe", "Smith");
pList[pList.Length - 1].address = "....";

Nolonar · Answer 5 · 2013-02-01T16:25:22.540

1

Should you have lost track of the element you're looking for in your list, you can always use LINQ to find the element again:

pList.First(person=>person.firstName == "John").lastName = "Doe";

Or if you need to relocate all "Doe"s at once, you can do:

foreach (Person person in pList.Where(p=>p.lastName == "Doe"))
{
    person.address = "Niflheim";
}

edited Feb 01 '13 at 16:25

answered Feb 01 '13 at 16:19

Nolonar

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How do I manipulate an object's properties after it has been added to a List in C#

5 Answers5