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

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);

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>.

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]);