IDictionary<string, string> dict = new Dictionary<string, string>();
dict.Add("MyKey", "N");
context.ConversationData.SetValue("MyDictionary", dict);

IDictionary<string, string> dict;
context.ConversationData.TryGetValue("MyDictionary", out dict)
var found = dict.ContainsKey("MyKey");

public static void Register(HttpConfiguration config)
{
    // Json settings
    config.Formatters.JsonFormatter.SerializerSettings.NullValueHandling = NullValueHandling.Ignore;
    config.Formatters.JsonFormatter.SerializerSettings.ContractResolver = new CamelCasePropertyNamesContractResolver { NamingStrategy = new CamelCaseNamingStrategy { ProcessDictionaryKeys = false } };
    config.Formatters.JsonFormatter.SerializerSettings.Formatting = Formatting.Indented;
    JsonConvert.DefaultSettings = () => new JsonSerializerSettings()
    {
        ContractResolver = new CamelCasePropertyNamesContractResolver { NamingStrategy = new CamelCaseNamingStrategy { ProcessDictionaryKeys = false } },
        Formatting = Formatting.Indented,
        NullValueHandling = NullValueHandling.Ignore,
    };

//...
}