automapper - 为什么我在使用 Condition 时会收到 "Nullable object must have a value"异常？

Question

在下面的示例代码中，我得到一个“可空对象必须有一个值”。为什么我需要以下修复来完成这项工作:

.ForMember(dest => dest.ShirtColor,
           dest => dest.MapFrom(src => src.ShirtColor != null
                                       ? new OptionSetValue((int) src.ShirtColor)
                                       : null))

AutoMapper.Mapper.CreateMap<PersonA, PersonB>()
    .ForMember(dest => dest.FirstName, dest => dest.MapFrom(src => src.FirstName))
    .ForMember(dest => dest.LastName, dest => dest.MapFrom(src => src.LastName))

    // Condition to avoid overwriting existing data!!!
    .ForMember(dest => dest.ShirtColor,
               dest => dest.Condition(src => src.ShirtColor != null))
    .ForMember(dest => dest.ShirtColor,
               dest => dest.MapFrom(
                   src => new OptionSetValue((int)src.ShirtColor)))
    // Fix that should not be needed due to condition:
    //.ForMember(dest => dest.ShirtColor,
    //           dest => dest.MapFrom(
    //               src => src.ShirtColor != null
    //                      ? new OptionSetValue((int) src.ShirtColor)
    //                      : null));


PersonA source = new PersonA();
source.FirstName = "Thomas";
source.LastName = "Jefferson";
source.ShirtColor = null;  // nullable int

PersonB destination = new PersonB();
destination.FirstName = "Thomas";
destination.LastName = "Jefferson";
destination.ShirtColor = new OptionSetValue(4);

// Results in: "Nullable object must have a value" despite the fact that
// condition should have been met!
Mapper.Map<PersonA, PersonB>(source, destination);

Debug.Assert(destination.ShirtColor != null);
Console.WriteLine("Our existing data was not overwritten!!");

Console.WriteLine("Hit enter to exit");
Console.ReadLine();

下面是如何定义 OptionSet:
公共(public)类 OptionSetValue
{
公共(public) OptionSetValue(){}

    public OptionSetValue(int value)
    {
        Number = value;
    }

    public int Number { get; set; }
}

Answer 1

只需使用 PreCondition而不是 Condition ..

所以改变你的

//Condition to avoid overwriting existing data!!!
.ForMember(dest => dest.ShirtColor, dest => dest.Condition(src => src.ShirtColor != null))
    .ForMember(dest => dest.ShirtColor, dest => dest.MapFrom(src => new OptionSetValue((int)src.ShirtColor)))

到

//Condition to avoid overwriting existing data!!!
.ForMember(dest => dest.ShirtColor, dest => dest.PreCondition(src => src.ShirtColor != null))
    .ForMember(dest => dest.ShirtColor, dest => dest.MapFrom(src => new OptionSetValue((int)src.ShirtColor)))