使用AutoMapper的ConvertUsing映射null字符串时自定义逻辑未执行问题

我正在使用AutoMapper的ConvertUsing(Func<TSource, TDestination, TDestination>)方法实现字符串到枚举的映射。但发现当调用mapper.Map<InternalDeliveryType>(string source)时，如果source为null，自定义的映射函数不会执行，结果直接返回枚举的默认值。

完整代码示例如下：

var mapperCfg = new MapperConfiguration(cfg =>
{
    cfg.CreateMap<string, InternalDeliveryType>()
    .ConvertUsing((s, d) =>
    {
        return s switch
        {
            null => InternalDeliveryType.Electronic,
            ExternalDeliveryType.Electronic => InternalDeliveryType.Electronic,
            ExternalDeliveryType.Paper => InternalDeliveryType.Paper,

            _ => throw new NotImplementedException($"Mapping '{s}' to {nameof(InternalDeliveryType)}.")
        };
    });
});

var mapper = mapperCfg.CreateMapper();

string? externalDeliveryType = null; // 从外部API传入
var internalDeliveryType = mapper.Map<InternalDeliveryType>(externalDeliveryType);
Debug.Assert(internalDeliveryType == InternalDeliveryType.Electronic);

内容的提问来源于Stack Exchange，提问作者Craig Boland