问题描述
我创建了一个包含一个对象的列表，其中包含一个对象列表。我想从父类列表/对象中的继承/子类对象中检索值。解释我下面的代码有点令人困惑，应该会有所帮助。也许 Linq 可能是解决方案？我不确定。非常感谢。
RelocationActivityForm 类 -- 这将使用继承的 Relocation 活动类创建一个新对象。
private Vehicle selectedVehicle;
selectedVehicle = (Vehicle)mainFormRego.SelectedItem; // Gets the selected item from a listBox
selectedVehicle.ActivityList.Add(new Relocation() // create an object for the selected vehicle,inside the activitylist (list) creating a new object using the Relocation class.
{
Date = date,// In activity class
Name = name,// In activity class
Type = "Relocation",// In activity class
Start = start,// In relocation class
End = end,// In relocation class
distance = distance,// In relocation class
Cost = cost // In activity class
});
ViewVehicleForm.Form.updateViewVehicle(); // Method updates textBox display.
Hide(); // Form hidden
}
车辆等级
namespace VehicleSystem
{
[Serializable]
public class Vehicle
{
private int dailyCharge;
private int year;
private string model;
private string make;
private string rego;
private List<Activity> _activityList = new List<Activity>();
public string Rego { get => rego; set => rego = value; }
public string Make { get => make; set => make = value; }
public string Model { get => model; set => model = value; }
public int Year { get => year; set => year = value; }
public int DailyCharge { get => dailyCharge; set => dailyCharge = value; } // Creating an ativity list for each vehicle
internal List<Activity> ActivityList { get => _activityList; set => _activityList = value;
}
}
}
活动类
[Serializable]
public class Activity
{
// Relocation Activity
private DateTime _date;
private string _name;
private string _type;
private decimal _cost;
public string Name { get => _name; set => _name = value; }
public string Type { get => _type; set => _type = value; }
public decimal Cost { get => _cost; set => _cost = value; }
public DateTime Date { get => _date; set => _date = value; }
}
搬迁班
class Relocation : Activity
{
private string start;
private string end;
private int distance;
public string Start { get => start; set => start = value; }
public string End { get => end; set => end = value; }
public int distance { get => distance; set => distance = value; }
}
使用它，我想从车辆的 ActivityList 中获取 Relocation 类中的项目。我可以从 ActivityList 中获取项目（通过调用此语句如下所示），但我无法获取使用 Relocation 类存储的活动/项目。
private void viewVehicleActivityData_CellContentClick(object sender,DataGridViewCellEventArgs e)
{
int x = viewVehicleActivityData.CurrentCell.RowIndex; // Getting the datagridview index of a row
MessageBox.Show(selectedVehicle.ActivityList[x].Name); // Using it to get the name
}
解决方法
是因为您的 ActivityList 是 List
确保在投射前进行适当的检查，请参见此处的示例：https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/operators/type-testing-and-cast
示例：
if (selectedVehicle.ActivityList[x] is Relocation relocation) {
MessageBox.Show(relocation.End);
}
您可以在此处使用各种实现。我确实认为@ebv 的回答和对类型转换的引用涵盖了一般概念，但我会为您的场景提供一个片段。 （参考您评论中的“我如何获得 End”）
此答案的实现将使用 as operator
哪个——
将表达式的结果显式转换为给定的引用或可为空的值类型
如果转换失败，它将返回 null。
因此，我们将使用 null-conditional operator（即 ?）和 null-coalescing operators (right-associative)（即 ??）来处理 null转换，否则如果发生转换，则返回 End。
private void viewVehicleActivityData_CellContentClick(object sender,DataGridViewCellEventArgs e)
{
int x = viewVehicleActivityData.CurrentCell.RowIndex; // Getting the datagridview index of a row
MessageBox.Show((selectedVehicle.ActivityList[x] as Relocation)?.End ?? "Activity is not Relocation"); // Using it to get the name
}
或者
在您给定的示例片段中，所有活动都是 Relocation 类型。
如果确实如此，您只需要在此处进行隐式转换 —
private void viewVehicleActivityData_CellContentClick(object sender,DataGridViewCellEventArgs e)
{
int x = viewVehicleActivityData.CurrentCell.RowIndex; // Getting the datagridview index of a row
MessageBox.Show(((Relocation)selectedVehicle.ActivityList[x]).End); // Using it to get the name
}