What code will insert a string into a List that is in a class in C# [duplicate]

Question

I am not experienced in C# I am trying to serialize some classes into a json string Here is an example of the json string I am trying to output and you can see the description is an array of lines

{
    "activities": [
        {
            "userName": "Gavin",
            "activity": {
                "id": "action001",
                "description": ["this is a long description"]
            }
        }
    ]
}

Here are the classes I have setup to model the json:

public class RootObject
    {
        [JsonProperty("activities")]
        public List<Activities> activities { get; set; }
    }

    public class Activities
    {
        [JsonProperty("userName")]
        public string UserName { get; set; }
        [JsonProperty("activity")]
        public _activity activity { get; set; }
    }

    
    public class _activity
    {
        [JsonProperty("id")]
        public string id { get; set; }
        [JsonProperty("description")]
        public List<string> Description { get; set; }
        
    }

Here is the C# code to create the example json:

var model = new RootObject();
            model.activities = new List<Activities>
            {
                new Activities
                {
                    UserName = "Gavin"
                    activity = new _activity

                    {
                        id = "action001",
                        Description[0] = "long description"
                    }, 
                }

The problem is that when I code Description[0] = "long description" I get an error in Visual Studio saying "Invalid initializer member declarator"

I would appreciate it if someone could show me the C# code I need in order to populate the Description list. Thank you

Answer 1

Create a string list as:

Description = new List<string> { "long description" }

var model = new RootObject();
model.activities = new List<Activities>
{
    new Activities
    {
        UserName = "Gavin"
        activity = new _activity
        {
            id = "action001",
            Description = new List<string> { "long description" }
        }, 
    }
};

Answer 2

The correct code is as follows:

public class RootObject
{
    [JsonProperty("activities")]
    public List<Activities> Activities { get; set; } = new List<Activities>();
}

public class Activities
{
    [JsonProperty("userName")]
    public string UserName { get; set; }

    [JsonProperty("activity")]
    public Activity Activity { get; set; } = new Activity();
}


public class Activity
{
    [JsonProperty("id")]
    public string Id { get; set; }

    [JsonProperty("description")]
    public List<string> Description { get; set; } = new List<string>();

}

var activity = new Activity
{
    Id = "action001"
};
activity.Description.Add("long description");

var model = new RootObject()
{
    Activities = new List<Activities>()
    {
        new Activities()
        {
            UserName ="Gavin",
            Activity = activity
        }
    }
};

Reference

https://docs.microsoft.com/dotnet/csharp/programming-guide/classes-and-structs/how-to-initialize-objects-by-using-an-object-initializer

https://docs.microsoft.com/dotnet/csharp/fundamentals/coding-style/coding-conventions

*