IQueryable where子句被继承类重写

Question

我想给继承类提供一个where子句给我的查询的机会。这可能吗？

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 
       }; 
    } 

先谢谢了!!!!!!!!!!

Answer 1

您可以定义可以申请额外的虚拟方法，其中条件：

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) 
{ 

} 

在你的派生类，你会那么做：

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

Answer 2

Where子句是一个扩展方法，因此您不能覆盖它，但可以将动态构造的表达式树传递给where子句并生成所需的结果。 看看这个MSDN文章在：

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

Answer 3

既然你返回一个IQueryable当使用资源的查询才会执行。所以只要它保持IQueryable，它将保留查询。

有了这些知识，你可以简单地套用在你的函数返回的IQueryable

像这样的地方：

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

由于OtherCOlumn心不是有你所说的，你可以更改默认查询：

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(); 
} 

然后在你的扩展位置做选择。由于它保持查询只要返回类型保持IQueryable这不会让它变得更慢

Answer 4

我不知道如何写Linq表示法，因为我总是喜欢使用Linq扩展方法，但它会看起来是这样的：

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"); 
} 

，或者通过提取其中（）条件的LINQ表达式：

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"; 
}