使用Linq表達式作爲具有父/子查詢的規範模式

Question

我試圖使用作爲Linq表達式實現的規範模式，以便Linq提供者可以解析它以生成高效的數據庫查詢。

This給出了基本思路。

我有一個艱難的時間，試圖設法得到它與父/子查詢

class Parent 
{ 
    public int Foo; 

    public IList<Child> Children = new List<Child>(); 
} 

class Child 
{ 
    public int Bar; 
} 

class Program 
{ 
    static void Main(string[] args) 
    { 
     IQueryable<Parent> qry = GetQry(); //initialised 


     //This works but duplicates the IsBigBar() logic 
     //Included to show what I am trying to query on 
     var parentsWithBigChildBars = 
       from parents in qry 
       where parents.Children.Any(child => child.Bar > 10) 
       select parents; 

     var parentsWithBigChildBars2 = 
       from parents in qry 
       where parents.Children.Any(??) //but how do i access my IsBigBar() expression from here? 
       select parents; 
    } 


    //I want to re-use it to pull parents back! 
    public Expression<Func<Child, bool>> IsBigBar() 
    { 
     return child => child.Bar > 10; 
    } 

    //I'f i use this as the Any() delegate, it compiles & runs but not an expression so evaluated client side 
    public Func<Child, bool> IsBigBar2() 
    { 
     return child => child.Bar > 10; 
    } 
} 

Answer 1

你想工作：

var predicate = IsBigBar(); 
    var parentsWithBigChildBars2 = 
      from parents in qry 
      where parents.Children.Any(predicate) 
      select parents; 

額外var是非常重要。它可以防止查詢提供程序（它擁有qry）試圖解釋IsBigBar()，而是將其指向該方法的結果。