Question

在最短的一到两周内，我们的代码中出现了一个奇怪的错误。我试图找出映射失败的根本原因。最内在的异常本身令人费解：Type 'System.String' does not have a default constructor

我不明白异常告诉我的是什么。你能解释一下发生了什么，也许我可以解决这个错误吗？

映射器在通用方法中使用：

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，快照不是null。完整的例外：

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; }
}

我仍在审核最近的提交，以确定导致此问题的更改。任何对异常的见解都将非常感激。

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创建了一个地图，取代了AutoMapper提供的默认映射。

如果您看到错误Type 'System.String' does not have a default constructor，请确认您的代码未为string创建地图。

Answer 2

String没有默认构造函数，因为字符串是不可变的，所以无论如何都不会有用。

要缩小您的问题范围，请向该方法提供哪些数据。您要映射的数据是什么。您要映射的项目中的Comment属性的值是多少？

Answer 3

我有同样的错误。我想将对象数组映射到对象数组

此生成的错误

AutoMapper.AutoMapperMappingException：输入'TargetObjectType'

没有默认构造函数：

AutoMapper.Mapper.CreateMap<SourceObjectType[], TargetObjectType[]>();
TargetObject = AutoMapper.Mapper.Map<SourceObjectType[], TargetObjectType[]>(SourceObject);

当我为映射设置正确的类型时，错误消失了。

AutoMapper.Mapper.CreateMap<SourceObjectType, TargetObjectType>();

AutoMapper.AutoMapperMappingException：类型'System.String'没有默认构造函数

3 个答案: