从动态集合创建地图不再有效：无法将 System.String 类型的对象转换为 System.Object）答案

【问题标题】：Create maps from dynamic collection no longer works: Unable to cast object of type System.String to System.Object)从动态集合创建地图不再有效：无法将 System.String 类型的对象转换为 System.Object）
【发布时间】：2020-04-14 20:26:11
【问题描述】：

我们正在将遗留代码移植到 dotnet 标准，并面临 [我相信] CsvHelper 2.5.0 net451 中不存在的问题。

问题似乎是 CsvReader 无法解析映射中基础属性的正确 ReturnType，并试图将它们全部映射到 System.Object

我已经设法在您的解决方案中通过单元测试重现了它：

[TestMethod]
public void CanCreateMapsFromDynamicList()
{
    var data = new List<string[]>
    {
        new[] { "Col1", "Col2" },
        new[] { "1", "one" },
        new[] { "2", "two" }
    };

    var queue = new Queue<string[]>(data);
    var parserMock = new ParserMock(queue);

    var csvReader = new CsvReader(parserMock);
    csvReader.Configuration.RegisterClassMap<SomeTypeClassMap>();

    var records = csvReader.GetRecords<SomeType>().ToList();

    Assert.IsNotNull(records);
    Assert.AreEqual(2, records.Count);

}

public class SomeTypeClassMap : ClassMap<SomeType>
{
    public SomeTypeClassMap()
    {
        Map(x => x.Id).Ignore();
        var t = new SomeType();

        foreach (var mapping in t.Mappings)
        {
            Map(mapping);
        }
    }
}

public class SomeType
{
    public int Id { get; set; }
    public string Col1 { get; set; }
    public string Col2 { get; set; }

    public IEnumerable<Expression<Func<SomeType, dynamic>>> Mappings =>
        new List<Expression<Func<SomeType, dynamic>>> {i => i.Col1, i => i.Col2};
}

报告错误：

测试方法 CsvHelper.Tests.CsvReaderMappingTestsRuslan.CanCreateMapsFromDynamicList 抛出异常：System.InvalidCastException：无法投射对象类型 'CsvHelper.Configuration.MemberMap2 [CsvHelper.Tests.CsvReaderMappingTestsRuslan+SomeType,System.String]' 输入 'CsvHelper.Configuration.MemberMap2[CsvHelper.Tests.CsvReaderMappingTestsRuslan+SomeType,System.Object]'。

附：我尝试将“映射”属性更改为静态并获得相同的结果。

【问题讨论】：

Op 已在 GitHub 项目上提交了一个错误问题。 github.com/JoshClose/CsvHelper/issues/1500

标签： c# .net-standard csvhelper csvreader

【解决方案1】：

通过添加这个重载方法解决它：

    public virtual MemberMap Map<T>( Expression<Func<T, object>> expression, bool useExistingMap = true )
    {
        var stack = ReflectionHelper.GetMembers(expression);
        if (stack.Count == 0)
        {
            throw new InvalidOperationException("No members were found in expression '{expression}'.");
        }

        ClassMap currentClassMap = this;
        MemberInfo member;

        if (stack.Count > 1)
        {
            // We need to add a reference map for every sub member.
            while (stack.Count > 1)
            {
                member = stack.Pop();
                Type mapType;
                var property = member as PropertyInfo;
                var field = member as FieldInfo;
                if (property != null)
                {
                    mapType = typeof(DefaultClassMap<>).MakeGenericType(property.PropertyType);
                }
                else if (field != null)
                {
                    mapType = typeof(DefaultClassMap<>).MakeGenericType(field.FieldType);
                }
                else
                {
                    throw new InvalidOperationException("The given expression was not a property or a field.");
                }

                var referenceMap = currentClassMap.References(mapType, member);
                currentClassMap = referenceMap.Data.Mapping;
            }
        }

        // Add the member map to the last reference map.
        member = stack.Pop();

        return currentClassMap.Map( typeof(TClass), member, useExistingMap );
    }

公关提出https://github.com/JoshClose/CsvHelper/pull/1505

【讨论】：