c# - AutoMapper.AutoMapperMappingException : Type 'System.String' does not have a default constructor 问题

Question

在最近一两周内，我们的代码中开始出现一个奇怪的错误。我正在尝试确定映射失败的根本原因。最内部的异常本身就令人费解:Type 'System.String' does not have a default constructor

我不明白异常告诉我什么。你能解释一下发生了什么吗？也许我可以如何解决这个错误？

映射器在泛型方法中使用:

public TEntity UpdateWithHistory<TEntity>(TEntity entity, int? entityID, int? interviewID)
where TEntity : class
{
    var snapshot = _interviewContext.Find<TEntity>(entityID);

    // This is call that fails
    var history = Mapper.Map<TEntity, TEntity>(snapshot);

    _interviewHistory.Set<TEntity>().Add(history);
    MarkModified(entity);
    return Mapper.Map(entity, snapshot);
}

在上面的代码中，snapshot 不为空。完整的异常:

AutoMapper.AutoMapperMappingException:
Trying to map Recog.Web.Models.InterviewComment to Recog.Web.Models.InterviewComment.
Using mapping configuration for Recog.Web.Models.InterviewComment to Recog.Web.Models.InterviewComment
Exception of type 'AutoMapper.AutoMapperMappingException' was thrown.
  ---> AutoMapper.AutoMapperMappingException: Trying to map System.String to System.String.
       Using mapping configuration for System.String to System.String
       Destination property: Comment
       Exception of type 'AutoMapper.AutoMapperMappingException' was thrown.
  ---> AutoMapper.AutoMapperMappingException: Trying to map System.String to System.String.
       Using mapping configuration for System.String to System.String
       Destination property: Comment
       Exception of type 'AutoMapper.AutoMapperMappingException' was thrown.
  ---> System.ArgumentException: Type 'System.String' does not have a default constructor
     at System.Linq.Expressions.Expression.New(Type type)
     at AutoMapper.DelegateFactory.CreateCtor(Type type)
     at AutoMapper.Mappers.ObjectCreator.CreateObject(Type type)
     at AutoMapper.MappingEngine.AutoMapper.IMappingEngineRunner.CreateObject(ResolutionContext context)
     at AutoMapper.Mappers.TypeMapObjectMapperRegistry.NewObjectPropertyMapMappingStrategy.GetMappedObject(ResolutionContext context, IMappingEngineRunner mapper)
     at AutoMapper.Mappers.TypeMapObjectMapperRegistry.PropertyMapMappingStrategy.Map(ResolutionContext context, IMappingEngineRunner mapper)
     at AutoMapper.Mappers.TypeMapMapper.Map(ResolutionContext context, IMappingEngineRunner mapper)
     at AutoMapper.MappingEngine.AutoMapper.IMappingEngineRunner.Map(ResolutionContext context)
     --- End of inner exception stack trace ---
     at AutoMapper.MappingEngine.AutoMapper.IMappingEngineRunner.Map(ResolutionContext context)
     at AutoMapper.Mappers.TypeMapObjectMapperRegistry.PropertyMapMappingStrategy.MapPropertyValue(ResolutionContext context, IMappingEngineRunner mapper, Object mappedObject, PropertyMap propertyMap)
     --- End

提到的 Comment 类:

public class InterviewComment
{
    [Key]
    public int? InterviewCommentID { get; set; }

    [ForeignKey("Interview")]
    public int? InterviewID { get; set; }

    [CodeType(CodeTypeEnum.CommentSection)]
    public int? CommentSectionCodeID { get; set; }

    [CodeType(CodeTypeEnum.CommentSource)]
    public int? CommentSourceCodeID { get; set; }

    [Display(Name = "Comment")]
    [StringLength(int.MaxValue)]
    public string Comment { get; set; }

    [Include]
    [Association("Interview_1-*_InterviewComment", "InterviewID", "InterviewID", IsForeignKey = true)]
    public virtual Interview Interview { get; set; }

    [ReadOnly(true)]
    [ForeignKey("ModifiedByUser")]
    public virtual ApplicationUser ApplicationUser { get; set; }

    [ReadOnly(true)]
    public string UserName
    {
        get { return ApplicationUser != null ? ApplicationUser.GetDisplayName() : null; }
    }

    [ReadOnly(true)]
    public int CreatedByUser { get; set; }

    [ReadOnly(true)]
    public DateTime CreatedDateTime { get; set; }

    [ReadOnly(true)]
    public int ModifiedByUser { get; set; }

    [ReadOnly(true)]
    public DateTime ModifiedDateTime { get; set; }
}

我仍在审查最近的提交以确定导致此问题的更改。非常感谢任何对该异常的洞察。

Answer 1

错误的根本原因在于未共享的代码。我们有一个约定，为通过反射发现的特定类型配置映射。我们的算法错误地为 string 创建了一个映射，替换了 AutoMapper 提供的默认映射。

如果您看到错误 Type 'System.String' does not have a default constructor，请确认您的代码没有为 string 创建映射。