通用LINQ TO SQL查询 [英] Generic LINQ TO SQL Query

问题描述

可以说我有两个由设计者生成的L2S类，其中一个属性如下：

  public class A 
 {
 public bool IsActive {get; set;} 
} 
 
 public class B 
 {
 public bool IsActive {get; set;} 
} 
  

我有一个通用的DataAccess类，如下所示：

  public class DataAccessBase< T>其中T：class 
 {
 NWDataContext dataContext = new NWDataContext（）; 
 
公共IList< T> GetAllActive（）
 {
 return dataContext.GetTable< T>（）
 .where（T.IsActive == true）----->我该如何做这样的事情？ 
 .ToList< T>（）; 
 
 
 $ / code> 

现在从GetAllActive（）如何返回所有A或B类型的活动对象。我的猜测是，我必须使用反射来做到这一点，但我对反射非常陌生。任何人都可以指向正确的方向吗？

解决方案

您需要传递给 Enumerable.Where 是 Predicate< T> 。 Predicate< T> 是一个委托，它可以吃掉 T 的实例并返回一个bool;您可以将谓词想象为代表 true 或 false 关于Ť。

现在有一个更大的问题，就是除非你对 T 施加约束，编译器没有知道 T 具有名为 IsActive 的公共可读属性的方法。因此，你必须定义一个接口（或者一个基类）来实现（或者派生自 A 和 B ），并告诉 GetAllActive 这个接口（或基类）的实现（或派生自）的方法。您可以通过在 DataAccessBase 的定义中限制 T 来实现。因此：

  interface IIsActive {
 bool IsActive {get;组; } 
} 
 
 class A：IIsActive {
 public bool IsActive {get;组; } 
} 
 class B：IIsActive {
 public bool IsActive {get;组; } 
} 
 
 public class DataAccessBase< T>其中T：class，IIsActive {
 public IList< T> GetAllActive（）{
 return dataContext.GetTable< T>（）
 .Where（x => x.IsActive）
 .ToList（）; 
} 
} 
  

Lets say I have two L2S classes generated by the designer with one property as follows:

public class A
{
  public bool IsActive {get;set;}
}

public class B
{
  public bool IsActive {get;set;}
}

I have a generic DataAccess class as follows:

public class DataAccessBase<T> where T : class 
    {
        NWDataContext dataContext = new NWDataContext();

        public IList<T> GetAllActive()
        {
            return dataContext.GetTable<T>()
                   .where(T.IsActive == true) -----> How can i do something like this?
                   .ToList<T>();
        }
   }

Now from the GetAllActive() how can I return all active objects of either A or B type. My guess is that I have to use reflection to do this but I am very new to reflection. Can anyone point me in the right direction?

解决方案

What you need to pass to Enumerable.Where is a Predicate<T>. A Predicate<T> is a delegate that can eat instances of T and return a bool; you can think of a predicate as representing a property that is either true or false about instaces of T.

Now, there is a larger issue which is that unless you put a constraint on T, the compiler has no way of knowing that T has a publicly readable property named IsActive. Thus, you have to define an interface (or a base class) that both A and B implement (or derive from) and tell the method GetAllActive that T implements (or derives from) this interface (or base class). You can do this by constraining T in the definition of DataAccessBase. Thus:

interface IIsActive {
    bool IsActive { get; set; }
}

class A : IIsActive {
    public bool IsActive { get; set; }
}
class B : IIsActive {
    public bool IsActive { get; set; }
}

public class DataAccessBase<T> where T : class, IIsActive {
    public IList<T> GetAllActive() {
    return dataContext.GetTable<T>()
                      .Where(x => x.IsActive)
                      .ToList();
    }
}

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