LINQ動態情況

Question

我有這樣的方法：

private string GetLanguage(String currentLanguage, dynamic entity) 
{ 
    return (currentLanguage == "de" ? entity.language.German : 
    currentLanguage == "fr" ? entity.language.French : 
    currentLanguage == "en" ? entity.language.English : 
    entity.language.English); 
} 

我需要這個返回做這樣的事情的SQL語句的LINQ表達式：

select (case when [french] is null then [english] else [french] end) as language 
from 

我怎樣才能做到這一點？

韓國社交協會

編輯：

更具體：

我有這樣的事情：

myId = 1; 

from objects in context.Objects 
          where objects.Id== myId 
select new 
          { 
           MyObject = new Models.MyObjects() 
           { 
            Id = objects.ScrId, 
            Title = LanguageFactory.GetLabelLanguage(objects.Labels) 
           } 
          }).FirstOrDefault(); 

而且MyObject.Title必須有明確的語言（英語，法文，荷蘭文等）。 在SQL我下定決心，通過做這樣的事情：

select id, (case when [french] is null then [english] else [french] end) as language 
from table 

我如何做一個LINQ表達式可以被轉換爲類似於上面的SQL查詢？

Answer 1

您將無法在Linq-to-SQL中調用任意函數。但是你可以使用一個Expression，像這樣：

private Expression<Func<DbObject, MyObject>> GetObjectWithLanguage = 
    e => new MyObject 
    { 
     Id = e.ScrId, 
     Title = e.currentLanguage == "de" ? e.Labels.language.German : 
       e.currentLanguage == "fr" ? e.Labels.language.French : 
       e.currentLanguage == "en" ? e.Labels.language.English : 
       e.Labels.language.English 
    }; 

... 
var results = context.Objects.Where(e => e.Id == myId).Select(GetObjectWithLanguage); 

這將使用在實體本身currentLanguage屬性，以確定哪些返回。但是，如果你想通過currentLanguage從你的C＃代碼的參數，試試這個：

private Expression<Func<DbObject, MyObject>> GetObjectWithLanguage(string currentLanguage) 
{ 
    if (currentLanguage == "de") 
    { 
     return e => new MyObject 
     { 
      Id = e.ScrId, 
      Title = e.Labels.language.German 
     } 
    } 
    else if (currentLanguage == "fr") 
    { 
     return e => new MyObject 
     { 
      Id = e.ScrId, 
      Title = e.Labels.language.French 
     } 
    } 
    ... 
} 

var results = context.Objects.Where(e => e.Id == myId).Select(GetObjectsWithLanguage("de")); 

如果你想創建一個通用的方法，我建議你使用仿製藥，而不是dynamic參數，例如，假設你有這樣一個接口：

public interface ILanguageAware 
{ 
    string CurrentLanguage { get; } 
    string English { get; } 
    string French { get; } 
    string German { get; } 
} 

public class ObjectWithLanguage<T> where T : ILanguageAware 
{ 
    public T OriginalObject { get; set; } 
    string LanguageText { get; set; } 
} 

public Expression<Func<T, ObjectWithLanguage<T>>> GetObjectWithLanguage<T>() where T : ILanguageAware 
{ 
    return x => new ObjectWithLanguage<T> 
      { 
       OriginalObject = x, 
       LanguageText = x.CurrentLanguage == "de" ? x.German : 
           x.CurrentLanguage == "fr" ? x.French : 
           ... 
      } 
} 

// extension method for convenience 
public static IQueryable<ObjectWithLanguage<T>> SelectWithLanguage<T>(this IQueryable<T> queryable) 
{ 
    return queryable.Select(GetObjectWithLanguage<T>()); 
} 

var results = context.Objectss.Where(e => e.Id == myId).SelectWithLanguage();