使用CSVHelper序列化/反序列化Dictionary<TKey,TValue>遇异常求助

问题背景 #

我需要用CSVHelper读写Dictionary<int, Sequence>类型的CSV文件，但尝试后抛出了MissingMethodException异常，相关代码和异常信息如下：

写入代码片段 #

using var writer = new StreamWriter(_filePath);
using var csv = new CsvWriter(writer, CultureInfo.InvariantCulture);
if (_classMap != null)
{
    csv.Context.RegisterClassMap(_classMap);
}
csv.WriteHeader(typeof(Dictionary<TKey, TValue>));
csv.NextRecord();

foreach (var value in _store)
{
     csv.WriteRecord(new KeyValuePair<TKey,TValue>(value.Key,value.Value));
     csv.NextRecord();
}

读取代码片段 #

using var reader = new StreamReader(_filePath);
using var csv = new CsvReader(reader, CultureInfo.InvariantCulture);
_ = csv.Read();
_ = csv.ReadHeader();

if (_classMap != null)
{
    csv.Context.RegisterClassMap(_classMap);
}

while (csv.Read())
{
    var key = csv.GetField<TKey>(0);
    var value = csv.GetRecord<TValue>();

    _store[key] = value;
}

Sequence类定义 #

public class Sequence
{
    public int SequenceId { get; set; }
    public int NextId { get; set; }
}

抛出的异常 #

System.MissingMethodException: Constructor 'CsvHelper.Configuration.DefaultClassMap1[[System.Collections.Generic.KeyValuePair2[[System.Int32, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e09],[FourDBS.SmartMeter.Common.Managers.Sequence, Four.DBS.Meadow.SmartMeter.Common, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]], mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]()'

解决方案 #

这个异常是因为CSVHelper尝试为KeyValuePair<TKey,TValue>创建默认类映射，但KeyValuePair是结构体，没有无参构造函数，导致映射创建失败。可以通过以下两种方式修复：

方式一：手动读写字段（简单直接） #

修复写入逻辑

不需要依赖自动映射，手动定义表头并写入每个字段：

using var writer = new StreamWriter(_filePath);
using var csv = new CsvWriter(writer, CultureInfo.InvariantCulture);

// 写入自定义表头
csv.WriteField("Key");
csv.WriteField(nameof(Sequence.SequenceId));
csv.WriteField(nameof(Sequence.NextId));
csv.NextRecord();

// 遍历Dictionary写入每一行
foreach (var kvp in _store)
{
    csv.WriteField(kvp.Key);
    csv.WriteField(kvp.Value.SequenceId);
    csv.WriteField(kvp.Value.NextId);
    csv.NextRecord();
}

修复读取逻辑

对应写入格式，手动读取字段并构造对象：

using var reader = new StreamReader(_filePath);
using var csv = new CsvReader(reader, CultureInfo.InvariantCulture);
csv.Read();
csv.ReadHeader();

while (csv.Read())
{
    var key = csv.GetField<int>("Key");
    var sequence = new Sequence
    {
        SequenceId = csv.GetField<int>(nameof(Sequence.SequenceId)),
        NextId = csv.GetField<int>(nameof(Sequence.NextId))
    };
    _store[key] = sequence;
}

方式二：自定义类映射（可复用） #

为KeyValuePair<int, Sequence>创建自定义映射，明确指定每个字段的对应关系：

public class KeyValueSequenceMap : ClassMap<KeyValuePair<int, Sequence>>
{
    public KeyValueSequenceMap()
    {
        Map(m => m.Key).Name("Key");
        Map(m => m.Value.SequenceId).Name(nameof(Sequence.SequenceId));
        Map(m => m.Value.NextId).Name(nameof(Sequence.NextId));
    }
}

修复写入逻辑

注册自定义映射后直接写入键值对：

using var writer = new StreamWriter(_filePath);
using var csv = new CsvWriter(writer, CultureInfo.InvariantCulture);
csv.Context.RegisterClassMap<KeyValueSequenceMap>();

csv.WriteHeader<KeyValuePair<int, Sequence>>();
csv.NextRecord();

foreach (var kvp in _store)
{
    csv.WriteRecord(kvp);
    csv.NextRecord();
}

修复读取逻辑

注册相同的映射后读取成键值对：

using var reader = new StreamReader(_filePath);
using var csv = new CsvReader(reader, CultureInfo.InvariantCulture);
csv.Context.RegisterClassMap<KeyValueSequenceMap>();

csv.Read();
csv.ReadHeader();

while (csv.Read())
{
    var kvp = csv.GetRecord<KeyValuePair<int, Sequence>>();
    _store[kvp.Key] = kvp.Value;
}

核心原因 #

CSVHelper的默认类映射要求目标类型具备无参构造函数，但KeyValuePair<TKey,TValue>是结构体，仅提供带参构造函数，无法自动生成默认映射，因此抛出MissingMethodException。通过手动读写或自定义映射即可解决该问题。

内容的提问来源于stack exchange，提问作者John Kattenhorn