项目泛型类型到KeyValuePair

我敢肯定我想要做的是相当简单，但我挣扎。我只是想将IQueryable投影到键值对中。项目泛型类型到KeyValuePair

这是我的课的一部分：

public DropDownListActionResult(IQueryable<T> dataItems, Func<T, int> keySelector, Func<T, string> valueSelector, int? selectedID) 
{ 
    _dataItems = dataItems; 
    _keySelector = keySelector; 
    _valueSelector = valueSelector; 
    _selectedID = selectedID; 
} 

public ActionResult Result() 
{ 
    var items = _dataItems.Select(item => 
       new KeyValuePair<int, string>(_keySelector.Invoke(item), _valueSelector.Invoke(item))) 
       .OrderBy("Value") 
       .ToList(); 
    items.Insert(0, new KeyValuePair<int, string>(0, "")); 

    var list = new SelectList(items, "Key", "Value", _selectedID); 

    return new JsonResult { Data = list, JsonRequestBehavior = JsonRequestBehavior.AllowGet }; 
}

我调用它像这样从一个单元测试。

var _stubRepository = MockRepository.GenerateStub<IGalleryRepository>(); 
    _stubRepository.Stub(x => x.Mediums).Return((
     new[] { 
     new Medium { MediumID = 1, MediumDescription = "Medium A"}, 
     new Medium { MediumID = 2, MediumDescription = "Medium B"}, 
    }).AsQueryable()); 

    _controller = new MediumController(_stubRepository, null, null); 

    var z = _controller.PopulateDropDownList(-1) as DropDownListActionResult<Medium>; 
    var result = z.Result(); 
    Assert.IsInstanceOf<JsonResult>(result);

这是PopulateDropDownList方法

public ActionResult PopulateDropDownList(int? ID) 
{ 
    return new DropDownListActionResult<Medium>(_repository.Mediums, 
               x => x.MediumID, 
               x => x.MediumDescription, ID); 
}

当运行它传递没问题的单元测试。但是，当我执行对LINQ to SQL中，我得到的错误：

Method 'Int32 Invoke(L2S_DomainModel.Entities.Medium)' has no supported translation to SQL.

来源

2010-08-24 Simon Lomax

完全错误信息说什么。 LinqToSql生成器不知道如何将任意lambda表达式（_keySelector.Invoke）转换为实际的SQL。将使用枚举，但不可查询。考虑用Expression <>包装你的lambda表达式。 – 2010-08-24 17:15:40

我已经尝试过使用表达式，但似乎没有得到任何地方。然而，多花点功夫，我设法让事情奏效。见下文。 – 2010-08-25 07:57:00

public DropDownListActionResult(IQueryable<T> dataItems, Expression<Func<T, int>> keySelector, Expression<Func<T, string>> valueSelector, int? selectedID) 
{ 
    _dataItems = dataItems; 
    _keySelector = keySelector; 
    _valueSelector = valueSelector; 
    _selectedID = selectedID; 
} 

public ActionResult Result() 
{  
    var items = _dataItems.Select(item => 
       new KeyValuePair<int, string>(_keySelector.Compile().Invoke(item), _valueSelector.Compile().Invoke(item))) 
       .ToDictionary(item => item.Key, item => item.Value); 

    items.Add(0, ""); 

    var list = new SelectList(items.OrderBy(i => i.Value), "Key", "Value", _selectedID); 

    return new JsonResult { Data = list, JsonRequestBehavior = JsonRequestBehavior.AllowGet };  
}

来源

2010-08-24 19:18:20

项目泛型类型到KeyValuePair

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