如何修复“无法将类型 System.Collections.Generic.List<> 隐式转换为 System.Collections.Generic.List<>”

Question

谁能告诉我为什么我无法在 GetWorkOrdersByLocation 中返回 workOrders？这是我的控制器：

public IActionResult Index()
{
    return View();
}

public IActionResult Privacy()
{
    return View();
}

public IActionResult WorkOrders()
{
    GetWorkOrders();
    return View(); 
}

public IActionResult GetWorkOrders()
{
    List<WorkOrder> workOrders = new List<WorkOrder> ();

    workOrders.Add(new WorkOrder { UserId = 1, LocationId = 
"Philadelphia", Date = DateTime.Now, Reason = "Lights", IsActive = 
true });

    workOrders.Add(new WorkOrder { UserId = 2, LocationId = "Camden", 
Date = DateTime.MinValue, Reason = "Plumbing", IsActive = true });

    workOrders.Add(new WorkOrder { UserId = 3, LocationId = 
"Burlington", Date = DateTime.Now, Reason = "Water", IsActive = 
false });

    workOrders.Add(new WorkOrder { UserId = 4, LocationId = 
"Wilmington", Date = DateTime.MaxValue, Reason = "Lights", IsActive 
= true }); return View(workOrders); 

}

public IActionResult GetWorkOrdersByLocation()
{
    List<WorkOrder> workOrders = new List<WorkOrder>();

    workOrders = workOrders.Select(x => x.LocationId).ToList();

    return workOrders; 

}

我在 return 语句 return workOrders 上遇到错误。我不确定为什么这不起作用。我从我的模型类 WorkOrder 创建列表 WorkOrder 的新实例。现在我正在尝试列出所有位置。

Answer 1

您的代码有几个问题：

1. 在GetWorkOrdersByLocation 中，您定义了List<WorkOrder> workOrders 并尝试为其分配List<string>，因为LocationId 是字符串类型。
2. 在GetWorkOrdersByLocation 中，您只有一个空的workOrders 列表，因此任何对其进行的选择也将返回空。
3. WorkOrders 似乎与 GetWorkOrdersByLocation 类似，但创建一个视图时不提供任何数据，因为尽管您调用 GetWorkOrdersByLocation，但您不会对其返回的内容做任何事情。
4. 次要 (?) 一个，GetWorkOrdersByLocation 的命名与您返回工作订单位置时似乎想要的不完全匹配，而不是按位置（排序/分组？）的工作订单。

一般来说，您需要一种方法来提供您的数据。 GetWorkOrders 似乎是这样做的，但是您返回的是视图，而不是数据。此方法只应返回您的列表，然后由WorkOrders 和GetWorkOrdersByLocation 使用。

public IActionResult WorkOrders()
{
    // get the work order list and return a view on it
    return View(GetWorkOrders()); 
}

private List<WorkOrder> GetWorkOrders()
{
    List<WorkOrder> workOrders = new List<WorkOrder> ();

    workOrders.Add(new WorkOrder { UserId = 1, LocationId = 
"Philadelphia", Date = DateTime.Now, Reason = "Lights", IsActive = 
true });

    workOrders.Add(new WorkOrder { UserId = 2, LocationId = "Camden", 
Date = DateTime.MinValue, Reason = "Plumbing", IsActive = true });

    workOrders.Add(new WorkOrder { UserId = 3, LocationId = 
"Burlington", Date = DateTime.Now, Reason = "Water", IsActive = 
false });

    workOrders.Add(new WorkOrder { UserId = 4, LocationId = 
"Wilmington", Date = DateTime.MaxValue, Reason = "Lights", IsActive 
= true });

 // return the work orders to be used by your view methods
 return workOrders; 
}

public IActionResult GetWorkOrdersByLocation()
{
    // get your work order list and extract all locations
    var locations = GetWorkOrders().Select(x => x.LocationId).ToList();

    // return view on all locations
    return View(locations); 
}

如果您希望最后一个方法返回按位置排序的所有工单的视图，请执行以下操作：

public IActionResult GetWorkOrdersByLocation()
{
    // get your work order list and sort by location
    var sortedByLocation = GetWorkOrders().OrderBy(x => x.LocationId).ToList();

    // return view on all sorted work orders
    return View(sortedByLocation); 
}

Answer 2

GetWorkOrdersByLocation() 实际上并没有从任何地方得到任何workOrders，它只是有一个空的List<WorkOrder>(); - 也许调用一些东西（私人？）从某个地方得到一些然后return View (workOrders); 我删掉了你的一些代码从我的示例中只关注请求。

private List<WorkOrder> CreateWorkOrders()
{
    List<WorkOrder> workOrders = new List<WorkOrder> ();

    workOrders.Add(new WorkOrder { UserId = 1, LocationId = 
"Philadelphia", Date = DateTime.Now, Reason = "Lights", IsActive = 
true });
    workOrders.Add(new WorkOrder { UserId = 2, LocationId = "Camden", 
Date = DateTime.MinValue, Reason = "Plumbing", IsActive = true });
    workOrders.Add(new WorkOrder { UserId = 3, LocationId = 
"Burlington", Date = DateTime.Now, Reason = "Water", IsActive = 
false });
    workOrders.Add(new WorkOrder { UserId = 4, LocationId = 
"Wilmington", Date = DateTime.MaxValue, Reason = "Lights", IsActive 
= true }); 
    return workOrders; 
}

public IActionResult GetWorkOrdersByLocation(string locationid)
{
    var workOrders CreateWorkOrders();
    locationWorkOrders = workOrders.Where(x => x.LocationId == locationid).ToList();
    return View(locationWorkOrders); 
}

评论中的问题：“被列表”是的List<string> 在这种情况下是一种方式。您还可以为您的位置创建一个类，因为这些类通常是具有 ID、短名称（如州缩写）、长名称、描述等具有适当命名属性的单独事物。