JSON反序列化异常：ABCRequest接口请求体格式问题排查

问题场景 #

定义了如下UserRequest和ABCRequest模型类，调用POST接口时先后遇到两次异常：

模型代码 #

[Route("/somepath", Verbs = "POST")]
public class ABCRequest
{
    public List<UserRequest> Requests { get; set; }
}

public class UserRequest
{
    public int UserID { get; set; }
    public DateTime FromDate { get; set; }
    public DateTime ToDate { get; set; }
}

第一次请求与异常 #

提交数组格式的JSON请求体：

[
    {
      "UserID": 123,
      "FromDate": "2015-01-01T00:00:00",
      "ToDate": "2016-01-01T23:59:59"
    },
    {
      "UserID": 456,
      "FromDate": "2015-01-01T00:00:00",
      "ToDate": "2016-01-01T23:59:59"
    }
]

抛出反序列化异常：

Could not deserialize 'application/json'
Error: Type definitions should start with a '{', expecting serialized type 'ABCRequest', got string starting with: [, {, "UserId": 123,
"From",

第二次请求与异常 #

改用包裹对象的请求体：

{
  "Requests": [
    {
      "UserID": 123,
      "FromDate": "2015-01-01T00:00:00",
      "ToDate": "2016-01-01T23:59:59"
    },
    {
      "UserID": 456,
      "FromDate": "2015-01-01T00:00:00",
      "ToDate": "2016-01-01T23:59:59"
    }
  ]
}

但提示：Request Dto is not supported

接口Post方法代码 #

简化版：

public async Task<SomeResponse> Post(ABCRequest request) 
{ var responses = new List<someResponse>(); 
  foreach (var userRequest in request.Requests) 
  {
   // 业务代码
  }
}

完整代码：

public async Task<SomeResponse> Post(ABCRequestRequest request)
{
    var responses = new List<GetSomeResponse>();
    foreach (var userRequest in request.Requests)
    {
        var userSummaries = await someService.GetSummaryAsync(
            userRequest.UserID, userRequest.FromDate.ToString(),  userRequest.ToDate.ToString());

        var data = userSummaries
            .Where(data => data.SomeData != 0)
            .Select(data => data).ToList();

        var response = new GetSomeResponse
        {
            UserID = userRequest.UserID
        };
        responses.Add(response);
    }
    return new SomeResponse
    {
        Responses = responses
    };
}

问题原因与解决 #

核心错误 #

第二次请求的JSON格式是正确的，但Post方法的参数类型写错了：定义的请求DTO是ABCRequest，但完整方法中参数用的是ABCRequestRequest，两者不匹配，导致框架无法识别请求DTO，抛出“Request Dto is not supported”错误。

解决方案 #

将Post方法的参数类型修正为ABCRequest，与定义的模型类一致：

public async Task<SomeResponse> Post(ABCRequest request)
{
    var responses = new List<GetSomeResponse>();
    foreach (var userRequest in request.Requests)
    {
        var userSummaries = await someService.GetSummaryAsync(
            userRequest.UserID, userRequest.FromDate.ToString(),  userRequest.ToDate.ToString());

        var data = userSummaries
            .Where(data => data.SomeData != 0)
            .Select(data => data).ToList();

        var response = new GetSomeResponse
        {
            UserID = userRequest.UserID
        };
        responses.Add(response);
    }
    return new SomeResponse
    {
        Responses = responses
    };
}

补充说明 #

第一次请求的错误原因是：接口接收的是ABCRequest类型（包含Requests数组的对象），但直接提交了数组JSON，不符合目标类型的结构，因此反序列化失败。第二次的请求体格式已经匹配ABCRequest的结构，只要修正方法参数类型即可正常运行。

内容的提问来源于stack exchange，提问作者wax