如何正确配置int?'到' int '投影使用AutoMapper

我有一些麻烦，让这个正确工作。我有两个类:

public class TestClassA
{
    public int? NullableIntProperty { get; set; }
}
public class TestClassB
{
    public int NotNullableIntProperty { get; set; }
}

然后设置以下映射:

cfg.CreateMap<TestClassA, TestClassB>()
    .ForMember(dest => dest.NotNullableIntProperty,
               opt => opt.MapFrom(src => src.NullableIntProperty));
cfg.CreateMap<TestClassA, TestClassA>()
    .ForMember(dest => dest.NullableIntProperty,
               opt => opt.MapFrom(src => src.NullableIntProperty));
cfg.CreateMap<TestClassB, TestClassA>()
    .ForMember(dest => dest.NullableIntProperty,
               opt => opt.MapFrom(src => src.NotNullableIntProperty));
cfg.CreateMap<TestClassB, TestClassB>()
    .ForMember(dest => dest.NotNullableIntProperty,
               opt => opt.MapFrom(src => src.NotNullableIntProperty));

int? => int
int => int?
int => int
int? => int?

var testQueryableDest = testQueryableSrc.ProjectTo<...>(_mapper.ConfigurationProvider);

我希望在这个阶段不工作的唯一投影是TestClassA => TestClassB，因为我可以看到AutoMapper可能不知道在值为null的情况下如何处理int?。果然，事实就是如此。因此，我为int? => int设置了如下映射:

cfg.CreateMap<int?, int>()
    .ProjectUsing(src => src ?? default(int));

这就是事情变得奇怪的地方。当我添加这个映射时，TestClassB => TestClassB的映射甚至无法创建。它给出如下错误信息:

System类型的表达式。Int32'不能用于赋值类型'System.Nullable ' 1[System.Int32]'

我发现这条消息非常奇怪，因为TestClassB上根本没有int?。这是怎么回事?我觉得我一定是误解了AutoMapper需要如何处理这些投影。我意识到各种代码可能很难拼凑在一起，所以这里是整个测试类供参考:

[TestClass]
public class BasicTests
{
    private readonly IMapper _mapper;
    public BasicTests()
    {
        var config = new MapperConfiguration(cfg =>
        {
            cfg.CreateMap<int?, int>()
                .ProjectUsing(src => src ?? default(int));
            cfg.CreateMap<TestClassA, TestClassB>()
                .ForMember(dest => dest.IntProperty, opt => opt.MapFrom(src => src.NullableIntProperty));
            cfg.CreateMap<TestClassA, TestClassA>()
                .ForMember(dest => dest.NullableIntProperty, opt => opt.MapFrom(src => src.NullableIntProperty));
            cfg.CreateMap<TestClassB, TestClassA>()
                .ForMember(dest => dest.NullableIntProperty, opt => opt.MapFrom(src => src.IntProperty));
            cfg.CreateMap<TestClassB, TestClassB>()
                .ForMember(dest => dest.IntProperty, opt => opt.MapFrom(src => src.IntProperty));
        });
        _mapper = new Mapper(config);
    }
    [TestMethod]
    public void CanMapNullableIntToInt()
    {
        var testQueryableSource = new List<TestClassA>
        {
            new TestClassA
            {
                NullableIntProperty = null
            }
        }.AsQueryable();
        var testQueryableDestination = testQueryableSource.ProjectTo<TestClassB>(_mapper.ConfigurationProvider);
    }
    [TestMethod]
    public void CanMapNullableIntToNullableInt()
    {
        var testQueryableSource = new List<TestClassA>
        {
            new TestClassA
            {
                NullableIntProperty = null
            }
        }.AsQueryable();
        var testQueryableDestination = testQueryableSource.ProjectTo<TestClassA>(_mapper.ConfigurationProvider);
    }
    [TestMethod]
    public void CanMapIntToNullableInt()
    {
        var testQueryableSource = new List<TestClassB>
        {
            new TestClassB
            {
                IntProperty = 0
            }
        }.AsQueryable();
        var testQueryableDestination = testQueryableSource.ProjectTo<TestClassA>(_mapper.ConfigurationProvider);
    }
    [TestMethod]
    public void CanMapIntToInt()
    {
        var testQueryableSource = new List<TestClassB>
        {
            new TestClassB
            {
                IntProperty = 0
            }
        }.AsQueryable();
        var testQueryableDestination = testQueryableSource.ProjectTo<TestClassB>(_mapper.ConfigurationProvider);
    }
}