在LINQ to Entities中初始化强类型对象 [英] Initializing strongly typed objects in LINQ to Entities

问题描述

我有一个简单的老CLR对象，它本质上是两个实体框架对象的包装器，我这样做，所以我可以在MVC框架中将这个包装器对象传递给一个强类型的视图。我的foo包装类很简单：

  public class FooWrapper 
 {
 public FooWrapper（Foo f，酒吧b）
 {
 this.FooObject = f; 
 this.BarObject = b; 
} 
 
 public Foo FooObject {get;私人集合} 
 public Bar BarObject {get;私人集合} 
} 
  

到目前为止，我的ListFoosWithBars功能如下： p>

  public IEnumerable< FooWrapper> ListFoosWithBars（int userID）
 {
 IEnumerable< Bar> tempBar = ListBarsByUserID（userID）; 
 IEnumerable< FooWrapper>结果=（从f在_entities.FooSet 
加入b在tempBar上f.ID等于b.foos.ID 
选择新的FooWrapper（f，b））; 
返回结果; 
} 
  

这不起作用，因为LINQ to Entities显然不支持参数初始化抛出一个例外：仅在LINQ to Entities中支持无参数构造函数和初始值设置。我想知道是否有另一种方式来实现同样的结果？

解决方案

如果您向FooWrapper添加无参数的构造函数，然后使用对象初始化，如下所示：

  public IEnumerable< FooWrapper> ListFoosWithBars（int userID）
 {
 IEnumerable< Bar> tempBar = ListBarsByUserID（userID）; 
 
 IEnumerable< FooWrapper>结果=（
 from f in _entities.FooSet 
 join b in tempBar on f.ID equals b.foos.ID 
 select new FooWrapper（）
 {
 FooObject = f，
 BarObject = b 
}）; 
 
返回结果; 
} 
  

I have a plain old CLR object which is essentially a wrapper for two entity framework objects, I'm doing this so I can pass this wrapper object to a strongly typed view in the MVC framework. My foo wrapper class is very simple:

public class FooWrapper
{
    public FooWrapper(Foo f, Bar b)
    {
        this.FooObject = f;
        this.BarObject = b;
    }

    public Foo FooObject { get; private set; }
    public Bar BarObject { get; private set; }
}

What I have so far for my ListFoosWithBars function is as follows:

public IEnumerable<FooWrapper> ListFoosWithBars(int userID)
{
    IEnumerable<Bar> tempBar = ListBarsByUserID(userID);
    IEnumerable<FooWrapper> results = (from f in _entities.FooSet
                                       join b in tempBar on f.ID equals b.foos.ID
                                       select new FooWrapper(f, b));
    return results;
}

This doesn't work because evidently LINQ to Entities doesn't support parametrized initialization, an exception is thrown that says just that: "Only parameterless constructors and initializers are supported in LINQ to Entities." I was wondering if there is another way to achieve this same result?

解决方案

IF you add a parameterless constructor to your FooWrapper and then use object initialization instead, like so:

public IEnumerable<FooWrapper> ListFoosWithBars(int userID)
{
    IEnumerable<Bar> tempBar = ListBarsByUserID(userID);

    IEnumerable<FooWrapper> results = (
        from f in _entities.FooSet
        join b in tempBar on f.ID equals b.foos.ID
        select new FooWrapper()
        {
            FooObject = f, 
            BarObject = b
        });

    return results;
}

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