IQueryable where子句被繼承類重寫

Question

我想給繼承類提供一個where子句給我的查詢的機會。這可能嗎？

protected IQueryable<EntityResult> GetEntities(ETBDataContext pContext) 
    { 
     return from e in pContext.Entities 
       where e.StatusCode == "Published" 

       //is there a way to add a dynamic where clause here? 
       //I would like to ask the inherited class for it's input: 

       && e.OtherColumn == "OtherValue" // <-- like GetWhere(e)? 

       //without having to select the column 

       orderby e.PublishDate descending 

       select new EntityResult 
       { 
        Name = e.Name, 
        Link = e.Link 
       }; 
    } 

先謝謝了!!!!!!!!!!

Answer 1

您可以定義可以申請額外的虛擬方法，其中條件：

protected IQueryable<EntityResult> GetEntities(ETBDataContext pContext) 
{ 
    IQueryable<Entity> query = pContext.Entities 
     .Where(e => e.StatusCode == "Published"); 

    query = ApplyWhereClause(query); 

    return from e in query 
      orderby e.PublishDate descending 
      select new EntityResult 
       { 
        Name = e.Name, 
        Link = e.Link 
       }; 
    } 

protected virtual IQueryable<Entity> ApplyWhereClause(IQueryable<Entity> entities) 
{ 

} 

在你的派生類，你會那麼做：

protected override IQueryable<Entity> ApplyWhereClause(IQueryable<Entity> entities) 
{ 
    return entities.Where(/* insert your extra where clause here */); 
} 

Answer 2

Where子句是一個擴展方法，因此您不能覆蓋它，但可以將動態構造的表達式樹傳遞給where子句並生成所需的結果。 看看這個MSDN文章在：

http://msdn.microsoft.com/en-us/library/bb882637.aspx

Answer 3

既然你返回一個IQueryable當使用資源的查詢纔會執行。所以只要它保持IQueryable，它將保留查詢。

有了這些知識，你可以簡單地套用在你的函數返回的IQueryable

像這樣的地方：

myObject.GetEntities(myContextObject).Where(x => x.id == 5); 

由於OtherCOlumn心不是有你所說的，你可以更改默認查詢：

protected IQueryable<EntityResult> GetEntities(ETBDataContext pContext) 
{ 
    return (from e in pContext.Entities 
      where e.StatusCode == "Published" 

      //is there a way to add a dynamic where clause here? 
      //I would like to ask the inherited class for it's input: 

      && e.OtherColumn == "OtherValue" // <-- like GetWhere(e)? 

      //without having to select the column 

      orderby e.PublishDate descending 

      select e).FirstOrDefault(); 
} 

然後在你的擴展位置做選擇。由於它保持查詢只要返回類型保持IQueryable這不會讓它變得更慢

Answer 4

我不知道如何寫Linq表示法，因爲我總是喜歡使用Linq擴展方法，但它會看起來是這樣的：

protected IQueryable<EntityResult> GetEntities(ETBDataContext pContext) 
{ 
    var q = pContext.Entities 
     .Where(e => e.StatusCode == "Published"); 
    q = q.AddWhereCondition(q) 
     .OrderByDescending(e => e.PublishDate) 
     .Select(e => new EntityResult 
      { 
       Name = e.Name, 
       Link = e.Link 
      }); 
} 

protected virtual IQueryable<Entity> AddWhereCondition(IQueryable<Entity> query) 
{ 
    return query.Where(e => e.OtherColumn == "OtherValue"); 
} 

，或者通過提取其中（）條件的LINQ表達式：

protected IQueryable<EntityResult> GetEntities(ETBDataContext pContext) 
{ 
    var q = pContext.Entities 
     .Where(e => e.StatusCode == "Published") 
     .Where(e => GetWhereCondition(e)) 
     .OrderByDescending(e => e.PublishDate) 
     .Select(e => new EntityResult 
      { 
       Name = e.Name, 
       Link = e.Link 
      }); 
} 

protected virtual Expression<Func<Entity, bool>> GetWhereCondition(Entity e) 
{ 
    return e => e.OtherColumn == "OtherValue"; 
}