Question

我有此代码：

public class Customer
{
    Int32 id { get; set; } = 0;
    User user1 { get; set; } = null;
    User user2 { get; set; } = null;
}

/* ... */

using (MySqlConnection conn = new MySqlConnection(Costanti.connessione))
{
    conn.Open();
    MySqlCommand m = new MySqlCommand("
SELECT c1.*, u1.*, u2.*
FROM customers as c1
inner join utenti u1 on u1.customer_id = c1.id
inner join utenti u2 on u2.customer_id = c1.id
", conn);

    MySqlDataReader x = m.ExecuteReader();
    DataTable dataTable = new DataTable();
    dataTable.Load(x);
    String json_string = Newtonsoft.Json.JsonConvert.SerializeObject(dataTable);

    List<Customer> lista = new List<Customer>();
    Newtonsoft.Json.JsonConvert.PopulateObject(json_string, lista);
    conn.Close();
}

如何将select的c1.*字段映射到通用Customer customer_1，然后将u1.*和u2.*映射到customer_1的属性中？ Newtonsoft.Json.JsonConvert.PopulateObject不允许我这样做。

中间json_string如下：

[
   {
      "id":1,
      "id_user":8,
      "name":"manuel",
      "id_user1":2,
      "name1":"michael"
   },
   {
      "id":2,
      "id_user":3,
      "name":"friedrich",
      "id_user1":6,
      "name1":"antony"
   }
]

结果必须是由以下内容组成的列表：

Customer(with id=1)，带有User1(8,"manuel")和User2(2,"michael")；
Customer(with id=2)，以及User1(3,"friedrich")和User2(6,"antony")。

Answer 1

对PopulateObject()的调用不起作用的主要原因是您的c＃数据模型与JSON的模式不匹配。如果我使用 How to auto-generate a C# class file from a JSON object string 中的一种工具从JSON自动生成数据模型，则会得到：

public class RootObject
{
    public int id { get; set; }
    public int id_user { get; set; }
    public string name { get; set; }
    public int id_user1 { get; set; }
    public string name1 { get; set; }
}

这看起来不像您的Customer类。

PopulateObject()失败的第二个原因是，默认情况下，Json.NET仅填充 public 个成员，而您都是私有的。

要解决此问题，我建议完全跳过DataTable和json_string表示形式，并直接从IDataReader实现的MySqlDataReader接口构建列表：

var lista = x
    .SelectRows(r =>
        // Extract the row data by name into a flat object
        new
        {
            id = Convert.ToInt32(r["id"], NumberFormatInfo.InvariantInfo),
            id_user = Convert.ToInt32(r["id_user"], NumberFormatInfo.InvariantInfo),
            name = r["name"].ToString(),
            id_user1 = Convert.ToInt32(r["id_user1"], NumberFormatInfo.InvariantInfo),
            name1 = r["name1"].ToString(),
        })
    .Select(r =>
        // Convert the flat object to a `Customer`.
        new Customer
        {
            id = r.id,
            user1 = new User { Id = r.id_user, Name = r.name },
            user2 = new User { Id = r.id_user1, Name = r.name1 },
        })
        .ToList();

使用扩展方法：

public static class DataReaderExtensions
{
    // Adapted from this answer https://stackoverflow.com/a/1202973
    // To https://stackoverflow.com/questions/1202935/convert-rows-from-a-data-reader-into-typed-results
    // By https://stackoverflow.com/users/3043/joel-coehoorn
    public static IEnumerable<T> SelectRows<T>(this IDataReader reader, Func<IDataRecord, T> select)
    {
        while (reader.Read())
        {
            yield return select(reader);
        }
    }
}

这假设您的Customer数据模型现在看起来像：

public class Customer
{
    public Int32 id { get; set; }
    public User user1 { get; set; }
    public User user2 { get; set; }
}

public class User
{
    public Int32 Id { get; set; }
    public string Name { get; set; }
}

您还可以将SelectRows和Select调用合并为一个方法；为了清楚起见，我将它们分开。跳过DataTable和JSON表示应该比您当前的方法更简单，更高效。

使用模型数据读取器here演示小提琴。

从JSON填充多个对象

1 个答案: