C#中如何从动态增长的长JSON文件获取最新交易的DestinationTag

我现在在C#里用dynamic变量读取长JSON文件，但新交易生成导致交易条目增加时，这种方式没法获取最新交易的DestinationTag——因为dynamic绑定的是特定位置的数值。请问该怎么解决？

相关JSON示例片段：

{ 
  "result":"success", 
  "count":5, 
  "transactions":[ 
    { 
      "hash":"866B841C8C3B2D840497679FAC45A7D4341E12056AAA53F67471CA1B955FB58E", 
      "ledger_index":37860930, 
      "date":"2018-04-10T12:35:32+00:00", 
      "tx":{ 
        "TransactionType":"Payment", 
        "Flags":2147483648, 
        "SourceTag":0, 
        "Sequence":20, 
        "DestinationTag":0, 
        "LastLedgerSequence":37860958, 
        "Amount":"25000000", 
        "Fee":"12", 
        "SigningPubKey":"02917EB4353C539A2B0766EFAB1723E5BB744516C673FFCF21457DAD44BCECFE25", 
        "TxnSignature":"304402202FA9FF18A572ACA1735B54CF5A69E10398EAF7003F3573E402ABA09B404F765C0220773481B494A941CC0F5338836A6B71F5FA2499943C41D50205C8EF158651BD86", 
        "Account":"rLcqTPYFSJBWGmu2ZXh8JTYGgdXRPjWTrx", 
        "Destination":"rfginNsDts73fZtzp858dT6Y6U67Y1QB7GV" 
      }, 
      "meta":{ 
        "TransactionIndex":1, 
        "AffectedNodes":[ 
          { 
            "CreatedNode":{ 
              "LedgerEntryType":"AccountRoot", 
              "LedgerIndex":"1C75CD43D75614E00025102355ADCCCF6302FAD5B82DC85F5AAF5696E4E39EC4", 
              "NewFields":{ 
                "Sequence":1, 
                "Balance":"25000000", 
                "Account":"rfginNsDt73fZtzp858dT6Y6U6s7Y1QB7GV" 
              } 
            } 
          }, 
          { 
            "ModifiedNode":{ 
              "LedgerEntryType":"AccountRoot", 
              "PreviousTxnLgrSeq":37757840, 
              "PreviousTxnID":"C794D81D17DF1EA735200E50B9CEADDF19160841E1321F08FDB7C7C4D6BC41AF", 
              "LedgerIndex":"BE486CB1FED1B1805E4C42E8E96C73CFDA4E520C66BACEBBE441C1F8212D980E", 
              "PreviousFields":{ 
                "Sequence":20, 
                "Balance":"537421240" 
              }, 
              "FinalFields":{ 
                "Flags":0, 
                "Sequence":21, 
                "OwnerCount":0, 
                "Balance":"512421228", 
                "Account":"rLcqTPYFSJBWGmu2ZXh8JTYGgdXRPjWTrx" 
              } 
            } 
          } 
        ], 
        "TransactionResult":"tesSUCCESS", 
        "delivered_amount":"25000000" 
      } 
    }, 
    { 
      "hash":"86E5D976DDCBF97047C86996286751B23C4384A8B4F6A04E38A57F30F54749CC", 
      "ledger_index":38906860, 
      "date":"2018-05-25T10:04:22+00:00", 
      "tx":{ 
        "TransactionType":"Payment", 
        "Flags":2147483648, 
        "Sequence":28534, 
        "DestinationTag":2150, 
        "Amount":"25000000", 
        "Fee":"45", 
        "SigningPubKey":"", 
        "Account":"rG2uw1W9FXoiY4QiNqej5WeRxnoBEw4JbL", 
        "Destination":"rfginNsDt73fZtzp858dT6Y6U67Y1QB7GV", 
        "Signers":[ 
          { 
            "Signer":{ 
              "SigningPubKey":"032DCABB7A17A96E826435B461E474CB33C86C98B2281D304367FCFC6DE463A215", 
              "TxnSignature":"3045022100F9802AE2F7800D42233A5B63999C0BD7AE5B7D60D8244C949FF81EA39959B4BC02205E37DD331ACE2637F2BEE758B5AD642D259F4140E8DE53EF610B6F58745BCC05", 
              "Account":"rDc4zYyxMbXfUXNKw6zfM88RBUd9y8uPYX" 
            } 
          }, 
          { 
            "Signer":{ 
              "SigningPubKey":"03019752C3ABCB2BC89EC5FAEF55D72AE940820A30FBA079D2BB8CE8841174EF21", 
              "TxnSignature":"30450221009C7F9519912D494C9F009130927E1248BE6C2F877B1A913B49B8BFD926442F6802205377D1D242D07033DC6FDA8C93BC65638D636A87539F32A0D269AA9B537FCB0D", 
              "Account":"rKB5N7FxvcwHSYq4NCKjyMpjML1MznMrFc" 
            } 
          } 
        ] 
      }, 
      "meta":{ 
        "TransactionIndex":17, 
        "AffectedNodes":[ 
          { "ModifiedNode": {} }
        ]
      }
    }
  ]
}

用dynamic确实容易遇到这种绑定固定位置的问题，毕竟它是编译时不检查类型、运行时才解析的，没法自动适应数组长度变化。这里给你几个实用的解决思路：

1. 使用强类型模型（推荐） #

最稳妥的方式是把JSON结构映射成C#类，这样不管交易条目怎么增加，都能通过遍历集合拿到所有交易的DestinationTag，包括最新的。

首先根据你的JSON结构生成对应的类（可以用Visual Studio的「编辑 > 选择性粘贴 > 将JSON粘贴为类」功能快速生成）：

public class TransactionResponse
{
    public string result { get; set; }
    public int count { get; set; }
    public List<Transaction> transactions { get; set; }
}

public class Transaction
{
    public string hash { get; set; }
    public int ledger_index { get; set; }
    public string date { get; set; }
    public Tx tx { get; set; }
    public Meta meta { get; set; }
}

public class Tx
{
    public string TransactionType { get; set; }
    public long Flags { get; set; }
    public int? SourceTag { get; set; } // 用可空类型处理可能缺失的字段
    public int Sequence { get; set; }
    public int? DestinationTag { get; set; }
    public int? LastLedgerSequence { get; set; }
    public string Amount { get; set; }
    public string Fee { get; set; }
    public string SigningPubKey { get; set; }
    public string TxnSignature { get; set; }
    public string Account { get; set; }
    public string Destination { get; set; }
    public List<SignerWrapper> Signers { get; set; }
}

public class SignerWrapper
{
    public Signer Signer { get; set; }
}

public class Signer
{
    public string SigningPubKey { get; set; }
    public string TxnSignature { get; set; }
    public string Account { get; set; }
}

public class Meta
{
    public int TransactionIndex { get; set; }
    public List<AffectedNode> AffectedNodes { get; set; }
    public string TransactionResult { get; set; }
    public string delivered_amount { get; set; }
}

public class AffectedNode
{
    public CreatedNode CreatedNode { get; set; }
    public ModifiedNode ModifiedNode { get; set; }
}

public class CreatedNode
{
    public string LedgerEntryType { get; set; }
    public string LedgerIndex { get; set; }
    public NewFields NewFields { get; set; }
}

public class NewFields
{
    public int Sequence { get; set; }
    public string Balance { get; set; }
    public string Account { get; set; }
}

public class ModifiedNode
{
    public string LedgerEntryType { get; set; }
    public int? PreviousTxnLgrSeq { get; set; }
    public string PreviousTxnID { get; set; }
    public string LedgerIndex { get; set; }
    public PreviousFields PreviousFields { get; set; }
    public FinalFields FinalFields { get; set; }
}

public class PreviousFields
{
    public int Sequence { get; set; }
    public string Balance { get; set; }
}

public class FinalFields
{
    public int Flags { get; set; }
    public int Sequence { get; set; }
    public int OwnerCount { get; set; }
    public string Balance { get; set; }
    public string Account { get; set; }
}

然后用Newtonsoft.Json（或者System.Text.Json）反序列化，之后就能轻松遍历所有交易，拿到最新的DestinationTag：

// 假设jsonString是你的JSON内容
var response = JsonConvert.DeserializeObject<TransactionResponse>(jsonString);

// 获取所有交易的DestinationTag，包括最新的
var allDestinationTags = response.transactions
    .Where(t => t.tx.DestinationTag.HasValue)
    .Select(t => t.tx.DestinationTag.Value)
    .ToList();

// 如果要拿最新的那笔交易的DestinationTag（假设transactions数组最后一个是最新的）
var latestDestinationTag = response.transactions.LastOrDefault()?.tx.DestinationTag;

2. 使用JObject遍历（无需强类型） #

如果不想定义一堆类，可以用Newtonsoft.Json的JObject来动态解析，直接遍历transactions数组：

var jObject = JObject.Parse(jsonString);
var transactions = jObject["transactions"] as JArray;

// 遍历所有交易，提取DestinationTag
foreach (var txObj in transactions)
{
    var destinationTag = txObj["tx"]["DestinationTag"]?.Value<int>();
    if (destinationTag.HasValue)
    {
        // 处理这个值
        Console.WriteLine(destinationTag.Value);
    }
}

// 获取最新的交易的DestinationTag
var latestTx = transactions.LastOrDefault();
var latestTag = latestTx?["tx"]["DestinationTag"]?.Value<int>();

3. 避免直接绑定dynamic的索引 #

如果你一定要用dynamic，那不要直接写dynamic.transactions[0].tx.DestinationTag这种固定索引的代码，而是把它当成集合遍历：

dynamic response = JsonConvert.DeserializeObject(jsonString);
foreach (var tx in response.transactions)
{
    var tag = tx.tx.DestinationTag;
    if (tag != null)
    {
        // 处理tag
    }
}

// 拿最新的
var latestTx = response.transactions.LastOrDefault();
var latestTag = latestTx?.tx.DestinationTag;

这样不管transactions数组新增多少条目，都能遍历到所有元素，不会错过最新的交易。

内容的提问来源于stack exchange，提问作者Jitender