LINQ错误：方法无法识别

Question

我使用下面的代码编译没有问题，但是当我打电话的方法，我得到这个错误：

LINQ to Entities does not recognize the method 'System.String ToString()' method, and this method cannot be translated into a store expression.

public IEnumerable<string> GetAllCitiesOfCountry(int id) 
    { 
     var ad = from a in entities.Addresses 
       where a.CountryID == id 
       select a.City.Distinct().ToString(); 
     var fa = from b in entities.FacilityAddresses 
       where b.CountryID == id 
       select b.City.Distinct().ToString(); 
     return ad.Concat(fa).Distinct(); 
    } 

怎样才可以重新编写，以便上班？

Answer 1

什么类型是City？如果它已经是一个字符串，只需放下.Distinct().ToString()调用。如果是复杂类型，请从类型中选择城市名称。

更新：根据您的评论，您应该放弃Distint（）和ToString（）调用。城市名称集合的最终联盟应该为您提供独特的城市名称。

public IEnumerable<string> GetAllCitiesOfCountry(int id) 
{ 
    var ad = from a in entities.Addresses 
      where a.CountryID == id 
      select a.City; 
    var fa = from b in entities.FacilityAddresses 
      where b.CountryID == id 
      select b.City; 
    return ad.Union(fa); 
} 

Answer 2

更新 - 我认为这是你在找什么

public IEnumerable<string> GetAllCitiesOfCountry(int id) 
    { 
     var ad = from a in entities.Addresses 
       where a.CountryID == id 
       select a.City; 
     var fa = from b in entities.FacilityAddresses 
       where b.CountryID == id 
       select b.City; 
     return ad.Union(fa).Distinct(); 
    }