方法不能被翻譯成店表達

我看到這個代碼工作，LINQ到SQL，但是當我使用實體框架，它拋出這個錯誤：方法不能被翻譯成店表達

LINQ to Entities does not recognize the method 'System.Linq.IQueryable'1[MyProject.Models.CommunityFeatures] GetCommunityFeatures()' method, and this method cannot be translated into a store expression.`

的庫代碼是這樣的：

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是一個擴展IQueryable功能的List。

有人可以解釋爲什麼會發生此錯誤嗎？

來源

2010-10-02 Shawn Mclean

原因： 根據設計，LINQ to Entities需要整個LINQ查詢表達式轉換爲服務器查詢。在翻譯查詢之前，只有少數不相關的子表達式（查詢中的表達式不依賴於服務器的結果）在客戶端上進行評估。沒有已知翻譯的任意方法調用，例如GetHomeFeatures（）在這種情況下不受支持。
更具體地說，LINQ to Entities只支持無參數構造函數和初始化程序。

解決方案： 因此，要克服這個例外，你需要你的子查詢合併到主之一GetCommunityFeatures（）和GetHomeFeatures（）而不是從LINQ查詢中直接調用方法。此外，您嘗試使用其參數化構造函數實例化一個新實例LazyList，就像您在LINQ to SQL中所做的一樣。爲此，解決方案將切換到客戶端評估LINQ查詢（LINQ到對象）。這將要求您在調用LazyList構造函數之前調用您的LINQ to Entities查詢的AsEnumerable方法。

像這樣的東西應該工作：

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：請看看LINQ to Entities, what is not supported?獲取更多信息。也可以查看LINQ to Entities, Workarounds on what is not supported瞭解可能的解決方案的詳細討論。（這兩個鏈接都是緩存版本，因爲原始網站已關閉）

來源

2010-10-03 15:01:06

方法不能被翻譯成店表達

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