How to edit value data in a dictionary C#

Question

I have a dictionary of members where the key is a unique long ID and the value is an object which contains data on that members name surname and other forms of member details. Is there any way in C# that this can be done?

e.g

dictionary key holds memberID 0 member id 0 name is bob lives in Italy

bob moves to England

is there a way to update the dictionary in C# so that his entry now says he lives in England?

Without knowing the generic arguments to the dictionary type, and the types they represent, no, we can't help you. — cdhowie, Dec 11 '10 at 20:15

score 6 · Answer 1 · answered Dec 11 '10 at 20:18

Assuming that Member (or whatever) is a class, it's simple:

members[0].Country = "England";

You're just updating the object which the dictionary has a reference to. Just to step through it, it's equivalent to:

Member member = members[0];
member.Country = "England";

There's only one object representing Bob, and it doesn't matter how you retrieve it.

In fact, if you already have access to the instance of Member via a different variable, you don't need to use the dictionary at all:

// Assume this will fetch a reference to the same object as is referred
// to by members[0]...
Member bob = GetBob();
bob.Country = "England";

Console.WriteLine(members[0].Country); // Prints England

If Member is actually a struct... well, then I'd suggest rethinking your design, and making it a class instead :)

score 5 · Answer 2 · answered Dec 11 '10 at 20:18

5

For classes (at least, those that are mutable) this should be as simple as:

long theId = ...
yourDictionary[theId].Country = "England"; // fetch and mutate

For structs (which should be immutable; or also for immutable classes), you will need to fetch, re-create, and overwrite:

long theId = ...
var oldItem = yourDictionary[theId]; // fetch
var newItem = new SomeType(oldItem.Id, oldItem.Name, "England"); // re-create
yourDictionary[theId] = newItem; // overwrite

(obviously the re-create line needs tweaking to your particular objects)

In the evil evil world of mutable structs (see comments), you can mutate once it is in a variable:

long theId = ...
var item = yourDictionary[theId]; // fetch
item.Country = "England"; // mutate
yourDictionary[theId] = item; // overwrite

answered Dec 11 '10 at 20:18

Marc Gravell

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Just FYI, you don't need to recreate the struct if it is mutable. – cdhowie Dec 11 '10 at 20:19
4

@cdhowie sshhh. Don't tell people that mutable structs exist. The next person I see waving around a mutable struct is going to get *beaten* with it. – Marc Gravell Dec 11 '10 at 20:20
@cdhowie added some notes on that; that do? – Marc Gravell Dec 11 '10 at 20:22
I am fairly new to this so doing your steps very slowly will let you know when I try your suggestions. Will let you know how it turns out. Thank you for your time :) – MrFail Dec 11 '10 at 20:44
In some situations, mutable structs can be extremely useful for performance reasons, assuming you know how to use them correctly. – cdhowie Dec 11 '10 at 21:36

score 1 · Answer 3 · answered Dec 11 '10 at 20:19

1

dictionary[memberID].Location = "Italy";

answered Dec 11 '10 at 20:19

Michael S. Scherotter

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score 1 · Answer 4 · answered Dec 11 '10 at 20:24

Well, I can't outcode Marc or Jon but here's my entry: (I used City instead of Country but the concept is the same.)

using System;
using System.Collections.Generic;

public class MyClass
{
    public static void Main()
    {
        var dict = new Dictionary<int, Member>();
        dict.Add(123, new Member("Jonh"));
        dict.Add(908, new Member("Andy"));
        dict.Add(456, new Member("Sarah"));

        dict[456].City = "London";

        Console.WriteLine(dict[456].MemberName  + " " + dict[456].City);
        Console.ReadKey();
    }
}

public class Member
{
    public Member(string name) {MemberName = name; City="Austin";}
    public string MemberName { get; set; }
    public string City { get; set; }
    // etc...
}

How to edit value data in a dictionary C#

4 Answers4