How do I cast a string type to a class type ? here is the ERROR MESSAGE:- unable to cast object of type 'system.string' to type 'query'

public static Xxl.Query QueryBySize(string SizeNo)
        {
            Query result = new Query();
            result.SizeInfo.SizeCode = "Xxl";
            Xxl.Query result1 = new Xxl.Query()
            {
                SIZE = SizeNo
            };
          
            JsonSerializerOptions options = new JsonSerializerOptions
            {
                IgnoreNullValues = true
            };
            string str = JsonSerializer.Serialize<Xxl.Query>(result1, options);
            Console.WriteLine("Retrieved Details: " + str);
            byte[] bytes = Encoding.UTF8.GetBytes(str);
            result.Data.Content = Convert.ToBase64String(bytes);
            Xxl.Query xxl = (Xxl.Query)QueryRequest(result );

            return xxl;
        }

public class Query
    {
        [JsonPropertyName("itemName")]
        public ItemName ItemName { get; set; }

        [JsonPropertyName("result")]
        public string Result { get; set; }

        public Query()
        {
            ItemName = new ItemName();
        }
    }