在AutoMapper 8.0.0中，你可以使用MapFrom方法和Condition方法来忽略具有默认值的属性映射。

首先，你需要定义一个自定义解析器来处理属性映射。在解析器中，你可以根据属性的默认值来决定是否进行映射。以下是一个示例代码：

public class IgnoreIfDefaultResolver : IMemberValueResolver
{
    public object Resolve(object source, object destination, object sourceMember, object destinationMember, ResolutionContext context)
    {
        // 如果属性的值等于默认值，则返回null，表示忽略映射
        if (Equals(destinationMember, Activator.CreateInstance(destinationMember.GetType())))
        {
            return null;
        }

        // 否则，返回属性的值进行正常映射
        return destinationMember;
    }
}

然后，在进行映射配置时，你可以使用MapFrom方法来指定要映射的属性，使用Condition方法来指定是否应该忽略映射。以下是一个示例代码：

// 创建映射配置
var config = new MapperConfiguration(cfg =>
{
    cfg.CreateMap()
        .ForMember(dest => dest.Property1, opt => opt.MapFrom(src => src.Property1))
        .ForMember(dest => dest.Property2, opt => opt.MapFrom(src => src.Property2))
        .ForMember(dest => dest.Property3, opt => opt.MapFrom(src => src.Property3))
        .ForMember(dest => dest.Property4, opt => opt.MapFrom(src => src.Property4))
        .ForMember(dest => dest.Property5, opt => opt.MapFrom(src => src.Property5))
        .ForMember(dest => dest.Property6, opt => opt.MapFrom(src => src.Property6))
        .ForMember(dest => dest.Property7, opt => opt.MapFrom(src => src.Property7))
        .ForMember(dest => dest.Property8, opt => opt.MapFrom(src => src.Property8))
        .ForMember(dest => dest.Property9, opt => opt.MapFrom(src => src.Property9))
        .ForMember(dest => dest.Property10, opt => opt.MapFrom(src => src.Property10))
        .ForMember(dest => dest.Property11, opt => opt.MapFrom(src => src.Property11))
        .ForMember(dest => dest.Property12, opt => opt.MapFrom(src => src.Property12))
        .ForMember(dest => dest.Property13, opt => opt.MapFrom(src => src.Property13))
        .ForMember(dest => dest.Property14, opt => opt.MapFrom(src => src.Property14))
        .ForMember(dest => dest.Property15, opt => opt.MapFrom(src => src.Property15))
        .ForMember(dest => dest.Property16, opt => opt.MapFrom(src => src.Property16))
        .ForMember(dest => dest.Property17, opt => opt.MapFrom(src => src.Property17))
        .ForMember(dest => dest.Property18, opt => opt.MapFrom(src => src.Property18))
        .ForMember(dest => dest.Property19, opt => opt.MapFrom(src => src.Property19))
        .ForMember(dest => dest.Property20, opt => opt.MapFrom(src => src.Property20))
        .ForAllOtherMembers(opt => opt.MapFrom());
});

// 创建映射器
var mapper = config.CreateMapper();

// 进行映射
var destination = mapper.Map(source);

在上面的示例中，我们使用了ForAllOtherMembers方法来指定要映射的所有其他属性，并使用IgnoreIfDefaultResolver解析器来决定是否忽略属性映射。

请注意，上述示例中的SourceClass和DestinationClass是自定义类，你需要根据你的实际情况进行替换。同时，你还需要根据你的实际需求调整解析器中的逻辑。