我检查了一下，发现即使是在JSON中指定数组也可以工作：

using System;
using System.Collections.Generic;
using Newtonsoft.Json;

class UserClass
{
    public string _id { get; set; }
    public string first_name { get; set; }
    public string last_name { get; set; }
    public string mobile { get; set; }
    public string email { get; set; }
    public DateTime createdAt { get; set; }
    public DateTime updateAt { get; set; }
    public int __v { get; set; }
}

class UserList
{
    public List<UserClass> objs;
}

public class Program
{
    public static void Main(string[] args)
    {
        string response = "{'objs': [{'_id': 1, 'first_name': 'abc'}, {'_id': 2, 'first_name': 'def'}]}";
        var result = JsonConvert.DeserializeObject<UserList>(response);
        if (result != null)
        {
            foreach (UserClass u in result.objs)
            {
                Console.Write(u._id + ',');
                Console.WriteLine(u.first_name);
            }
        }
        string response2 = "[{'_id': 1, 'first_name': 'abc'}, {'_id': 2, 'first_name': 'def'}]";
        var result2 = JsonConvert.DeserializeObject<List<UserClass>>(response2);
        if (result2 != null)
        {
            for (var i = 0; i < result2.Count; i++)
            {
                Console.Write(result2[i]._id + ',');
                Console.WriteLine(result2[i].first_name);   
            }
        }
    }
}

两种方式给予输出相同。https://dotnetfiddle.net/g2Dn1n
我做的唯一附加的事情是检查返回值。您应该在Unity中使用Debug.Log()来打印反序列化后获得的实际值。

经过几天的努力，答案终于找到了
为了解决这个问题，我对代码进行了如下编辑：
我的代码：

socket.On("UserList", response =>
{
    var jsonParse = JToken.Parse(response.ToString());
    var jsonArray = jsonParse[0];
    var DSJson = JsonObject.Deserialise<List<UserClass>>(jsonArray.ToString());
    print(DSJson[0].first_name);
});

//输出：sara
用户类别：

public class UserClass
{
public string _id { get; set; }
public string first_name { get; set; }
public string last_name { get; set; }
public string mobile { get; set; }
public string email { get; set; }
public DateTime createdAt { get; set; }
public DateTime updateAt { get; set; }
public int __v { get; set; }
}

在客户端接收的Json：

[
{
  "_id": "83ca7d56cbc2b281wd4ee658",
  "first_name": "sara",
  "last_name": "Paulson",
  "mobile": "09323456789",
  "email": "sara@gmail.com",
  "createdAt": "2023-01-20T12:46:38.384Z",
  "updateAt": "2023-01-20T12:46:38.384Z",
  "__v": 0
},
{
  "_id": "59e41dku510239e83ed7e10m",
  "first_name": "Evan",
  "last_name": "Peters",
  "mobile": "09327931248",
  "email": "Evan@gmail.com",
  "createdAt": "2023-02-10T10:35:26.687Z",
  "updateAt": "2023-02-10T10:35:26.687Z",
  "__v": 0
},
{
  "_id": "64lm96c57a8a4f289fw0gg66",
  "first_name": "Emma",
  "last_name": "Roberts",
  "mobile": "09325354769",
  "email": "Emma@gmail.com",
  "createdAt": "2023-01-20T13:11:46.402Z",
  "updateAt": "2023-01-20T13:11:46.402Z",
  "__v": 0
}
  ]

这样，您就可以访问从服务器发送的json值