How to consume Moodle web service functions from c#?

I hava a task to create Moodle courses categories from my c# project, but the following code always return an error response : "Missing required key in single structure: categories" My code is as follows:

        string sitename = "http://localhost/moodle"; 
        string token = "0d31f83da49cc21f8dc599fab4b299f0"; 
        string url = sitename + "/webservice/rest/server.php?wstoken=" + token + "&wsfunction=core_course_create_categories&moodlewsrestformat=json"; 
        category cat = new category(); 
        List<category> categories = new List<category>(); 
        cat.name = HttpUtility.UrlEncode("2019"); 
        cat.parent = int.Parse(HttpUtility.UrlEncode("2")); 
        cat.idnumber = HttpUtility.UrlEncode("11"); 
        cat.description = HttpUtility.UrlEncode("Academic year 2019"); 
        cat.descriptionformat = int.Parse(HttpUtility.UrlEncode("1")); 
        cat.theme = HttpUtility.UrlEncode(""); 
        categories.Add(cat); 
        Dictionary<string, List<category>> param = new Dictionary<string, List<category>>(); 
        param.Add("categories", categories.ToList()); 
        string jsonList = JsonConvert.SerializeObject(param.ToArray(), Formatting.None); 
        var client = new RestClient(url); 
        var request = new RestRequest(); 
        request.AddHeader("Content-Type", "application/json"); 
        request.AddHeader("Accept", "application/json"); 
        request.RequestFormat = DataFormat.Json; 
        request.AddJsonBody(jsonList); 
        request.Method = Method.POST; 
        var response = client.Execute(request); 

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//category class code

public class category 
{ 
    public string name { get; set; } 
    public int parent { get; set; } 
    public string idnumber { get; set; } 
    public string description { get; set; } 
    public int descriptionformat { get; set; } 
    public string theme { get; set; } 
    public category() 
    { 
        descriptionformat = 1; 
        theme = ""; 
    } 
} 

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