How to check null values in dynamic JObject?

Question

I have a JObject variable UserPreferences (C#); It will contain dynamic key value pairs based on request.

It is guaranteed that the key exists. I just want to check if the key has an empty value so that I can throw a "Missing fields" exception following which user can then fill the fields and send request again. How can I achieve this (preferably with iteration over the keys present in UserPreferences object [below])? A controller receives the request and maps it to a User class object. How do I check if there is any empty value in any key?

My requests will be of the form:

{
 "UserName":"Mark",
 "UserPreferences":{
    "Section1":"Do",
    "Section2":"Delete"
  }
}

or it can also be like:

{
     "UserName":"Mark",
     "UserPreferences":{
        "Section1":"Do",
        "Section2":"Delete",
        "Section4":"Add_Name" 
      }
 }

Hence, the request can contain dynamic number of key value pairs in UserPreferences object (more sections and their values); But sometimes a user may leave the field empty. So, when the request arrives, it will look something like this:

 {
       "UserName":"Mark",
       "UserPreferences":{
          "Section1":"",
          "Section2":""
          }
  }

or it can look like this (another example):

 {
       "UserName":"Mark",
       "UserPreferences":{
          "Section1":"Do",
          "Section2":""
          }
  }

Here's what my class that handles the request looks like:

public class User
{
 public string User{get;set;}
 public JObject UserPreferences{get;set;}
}

Note that I do not want to use any other structure like dictionary as a complete replacement of the JObject structure used above.

Answer 1

Parse UserPreferences as Dictionary<string, string> and check dictionary value to be not null:

public class User
{
    public string UserName { get; set; }
    public Dictionary<string, string> UserPreferences { get; set; }
}

User u = ... // parse json
if(u.UserPreferences.Values.Any(v => v == null)) 
{
    ...
}