Serialize Dictionary<string, string> with key having object structure

In order to do what you want, now that you've provided a more full question, you will need to do the following steps:

1. For each key, extract the index number and the property name
2. Then, make sure you have a Contact object inside the Contacts list, for the [index] index, according to the index you found in point 1
3. Using the property name from the key, assign the value from the dictionary related to the key to the right property on the relevant Contact object

Here's a piece of code that will do this:

void Main()
{
    var dict = new Dictionary<string, string>
    {
        ["customer.contacts[0].adresse"] = "39 Dummy Street",
        ["customer.contacts[0].email"] = "[email protected]",
        ["customer.contacts[1].adresse"] = "455 Dummy Street",
        ["customer.contacts[1].email"] = "[email protected]",
        ["customer.contacts[2].adresse"] = "72 Dummy Street",
        ["customer.contacts[2].email"] = "[email protected]",
    };

    var c = new Customer { Contacts = new List<Customer.Contact>() };
    var re = new Regex(@"^customer.contacts\[(?<idx>\d+)\]\.(?<prop>.*)$");
    foreach (var (key, value) in dict)
    {
        var ma = re.Match(key);
        if (!ma.Success)
            continue;

        int index = int.Parse(ma.Groups["idx"].Value);
        string prop = ma.Groups["prop"].Value;

        while (index >= c.Contacts.Count)
            c.Contacts.Add(new Customer.Contact());

        switch (prop)
        {
            case "adresse":
                c.Contacts[index].Adresse = value;
                break;
                
            case "email":
                c.Contacts[index].Email = value;
                break;
        }
    }
    
    Console.WriteLine(JsonConvert.SerializeObject(c, Newtonsoft.Json.Formatting.Indented));
}

public class Customer
{
    public List<Contact> Contacts { get; set; }
    public class Contact
    {
        public string Adresse { get; set; }
        public string Email { get; set; }
    }
}

You can see it in action here on dotnetfiddle.

The output of the above will be:

{
  "Contacts": [
    {
      "Adresse": "39 Dummy Street",
      "Email": "[email protected]"
    },
    {
      "Adresse": "455 Dummy Street",
      "Email": "[email protected]"
    },
    {
      "Adresse": "72 Dummy Street",
      "Email": "[email protected]"
    }
  ]
}

Be aware of the following:

1. If your keys skip an index you will get empty Contact objects in the Contacts collection. There will be an instance there, but none of the properties have been set. Example, your dictionary has keys for [0], [1] and [3], but missing [2]. There will be a Contact object for [2], but it will have default property values.
2. There is no error checking that you're setting all the properties on any of the Contact objects. For instance, it is enough that you have a "customer.contacts[2].xyz" to get a Contact object for index [2], but property name "xyz" won't match anything, and if you don't have any other mentions of [2], both Adresse and Email will be left empty.