C# List<object> to Dictionary<key, <object>>

Question

I am a beginner c# developer and I need to create a dictionary of object from my list. First, let me define my object as Person.

public class Person 
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int Age { get; set; }
}

Now I have a list of my person object. List<Person> How to query it in LinQ to convert it to a Dictionary of Person from my list? My desired output is:

Dictionary<key, <Person>>

Where key is an incrementing integer per Person object.. Any help is appreciated. Thank you.

I found this code online but it works with List<string>

List<string> List1
var toDict = List1.Select((s, i) => new { s, i })
             .ToDictionary(x => x.i, x => x.s)

Solution 1

One most straight forward way would be to use the int key as key like this:

List<Person> List1 = new List<Person>();
int key = 0; //define this for giving new key everytime
var toDict = List1.Select(p => new { id = key++, person = p })
    .ToDictionary(x => x.id, x => x.person);

The key is the lambda expression:

p => new { id = key++, person = p }

Where you create an anonymous object having id and person properties. The id is incremental key while the person is simply the element of your List<Person>

If you need to use the Person's Id instead, simply use:

List<Person> List1 = new List<Person>();
var toDict = List1.Select(p => new { id = p.Id, person = p })
    .ToDictionary(x => x.id, x => x.person);

Solution 2

This should work for your case:

int key = 0; // Set your initial key value here.
var dictionary = persons.ToDictionary(p => key++);

Where persons is List<Person>.

Solution 3

You were almost there, just change variable type from List<string> to List<Person> and you are good to go. You can use your LINQ query as is, example:

List<Person> persons = new List<Person>();

var p1 = new Person();
p1.Name = "John";
persons.Add(p1);

var p2 = new Person();
p2.Name = "Mary";
persons.Add(p2);

var toDict = persons.Select((s, i) => new { s, i })
             .ToDictionary(x => x.i, x => x.s);

However, while I don't have anything against LINQ, in this particular case a much more readable approach is using a regular loop like this:

var result = new Dictionary<int, Person>();
for (int i = 0; i < persons.Count; i++)
{
    result.Add(i, persons[i]);
}

Jon Skeet suggests yet another way of doing it, using Enumerable.Range, which I tested and it works perfectly:

var toDict = Enumerable.Range(0, persons.Count)
             .ToDictionary(x => x, x => persons[x]);

Solution 4

This is my solution:

var key = 0;
IDictionary<int, Person> personDictionary = personList
    .Select(x => new KeyValuePair<int,Person>(key++, x))
    .ToDictionary(y => y.Key, y => y.Value);

where personList is List<Person>

View more solutions

Comments

[super-user]

Thank you for a very complete newbie-friendly answer. I will select this as the answer later when I can (answered too fast 5 more min)

[Ian]

@super-user no problem. :) it may take some times to learn. But hope you get it - I purposely put up some explanations for you to understand how the Lambda expression in the example you found work.

[Victor Zakharov]

You could easily win code golf challenge with this. +1

[super-user]

Thank you! Neat and short. +1 Sorry minute too late to be selected as the answer

[super-user]

thanks! works too, I appreciate the suggestions, will look into it. + 1

[Victor Zakharov]

@super-user: [whispers] You can always change your accepted answer. :) meta.stackexchange.com/questions/120568/…

[Ian]

@super-user as for more efficient way of doing that without going from your example, kindly check Seprum's answer.

[Abhishek]

the last ".Select" statment has an extra "(", which should be removed.