AutoMapper.AutoMapperMappingException: Type 'System.String' does not have a default constructor

Question

A strange error began occurring in our code in the lest week or two. I am trying to identify the root cause of the mapping failure. The most-inner exception itself is puzzling: Type 'System.String' does not have a default constructor

I don't understand what the exception is telling me. Can you explain what has happened and maybe how I could resolve this bug?

The mapper is used within a generic method:

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);
}

In the above code, snapshot is NOT null. The full exception:

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

The Comment class that is mentioned:

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; }
}

I'm still reviewing recent commits to identify the change that is causing this. Any insight into the exception would be greatly appreciated.

Answer 1

The root cause of the error was in code that was not shared. We have a convention that configured mappings for specific types discovered through reflection. Our algorithm incorrectly created a map for string, replacing the default mapping provided by AutoMapper.

Should you ever see the error Type 'System.String' does not have a default constructor, confirm your code does not create a map for string.