Question

我看到这段代码与LINQ to SQL一起使用，但是当我使用Entity Framework时，它会抛出此错误：

LINQ to Entities无法识别方法'System.Linq.IQueryable'1 [MyProject.Models.CommunityFeatures] GetCommunityFeatures（）'方法，而且此方法无法转换为商店表达式。

存储库代码是这样的：

public IQueryable<Models.Estate> GetEstates()
{
    return from e in entity.Estates
           let AllCommFeat = GetCommunityFeatures()
           let AllHomeFeat = GetHomeFeatures()
           select new Models.Estate
                      {
                                EstateId = e.EstateId,
                                AllHomeFeatures = new LazyList<HomeFeatures>(AllHomeFeat),
                                AllCommunityFeatures = new LazyList<CommunityFeatures>(AllCommFeat)
                      };
}

public IQueryable<Models.CommunityFeatures> GetCommunityFeatures()
{
    return from f in entity.CommunityFeatures
           select new CommunityFeatures
                      {
                          Name = f.CommunityFeature1,
                          CommunityFeatureId = f.CommunityFeatureId
                      };
}

public IQueryable<Models.HomeFeatures> GetHomeFeatures()
{
    return from f in entity.HomeFeatures
           select new HomeFeatures()
           {
               Name = f.HomeFeature1,
               HomeFeatureId = f.HomeFeatureId
           };
}

LazyList是一个扩展IQueryable功能的List。

有人可以解释为什么会出现这种错误吗？

Answer 1

<强>原因：按照设计， LINQ to Entities 要求将整个LINQ查询表达式转换为服务器查询。在翻译查询之前，仅在客户端上评估几个不相关的子表达式（查询中不依赖于服务器结果的表达式）。不支持没有已知转换的任意方法调用，例如本例中的GetHomeFeatures（）。
更具体地说，LINQ to Entities仅支持无参数构造函数和 Initializers 。

的解决方案：因此，要克服此异常，您需要将子查询合并到 GetCommunityFeatures（）和 GetHomeFeatures（）的主查询中，而不是直接从LINQ查询中调用方法。此外，您尝试使用其参数化构造函数实例化 LazyList 的新实例也存在问题，就像您在 LINQ to SQL 中所做的那样。为此，解决方案是切换到LINQ查询（LINQ to Objects）的客户端评估。这将要求您在调用LazyList构造函数之前为LINQ to Entities查询调用AsEnumerable方法。

这样的事情应该有效：

public IQueryable<Models.Estate> GetEstates() { return from e in entity.Estates.AsEnumerable() let AllCommFeat = from f in entity.CommunityFeatures select new CommunityFeatures { Name = f.CommunityFeature1, CommunityFeatureId = f.CommunityFeatureId }, let AllHomeFeat = from f in entity.HomeFeatures select new HomeFeatures() { Name = f.HomeFeature1, HomeFeatureId = f.HomeFeatureId }, select new Models.Estate { EstateId = e.EstateId, AllHomeFeatures = new LazyList<HomeFeatures>(AllHomeFeat), AllCommunityFeatures = new LazyList<CommunityFeatures>(AllCommFeat) }; }

更多信息：请查看 LINQ to Entities, what is not supported? 了解详情。另请查看 LINQ to Entities, Workarounds on what is not supported 以获取有关可能解决方案的详细讨论。（两个链接都是缓存版本，因为原始网站已关闭）

方法无法转换为商店表达式

1 个答案: