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[英]session {“Object reference not set to an instance of an object.”} System.Exception {System.NullReferenceException}
[英]System.NullReferenceException: „Object reference not set to an instance of an object.” problem
using System;
namespace zestaw_6
{
public class Program
{
public static void Main(string[] args)
{
var zoo = new Zoo();
zoo.Add(new Opiekun("Jan", "Kowalski"));
Console.ReadKey();
}
}
using System;
using System.Collections.Generic;
using System.Text;
using System.Linq;
namespace zestaw_6
{
public static class ActionExtensions
{
public static IList<TObj> Set<TObj, TAg>(this TAg aggregatedObj) where TObj : IInfo where TAg : IAction
{
var aggregatedObjType = aggregatedObj.GetType();
var propertyInfo = aggregatedObjType.GetProperties().FirstOrDefault(p => p.PropertyType == typeof(IList<TObj>));
var propertyValue = propertyInfo?.GetValue(aggregatedObj);
return propertyValue as IList<TObj>;
}
public static C Get<C>(this IAction container, Func<C, bool> searchPredicate = null) where C : IInfo
{
return searchPredicate == null ? container.Set<C, IAction>().FirstOrDefault() : container.Set<C, IAction>().FirstOrDefault(searchPredicate);
}
public static IList<C> GetList<C>(this IAction container, Func<C, bool> searchPredicate = null) where C : IInfo
{
return searchPredicate == null ? container.Set<C, IAction>() : container.Set<C, IAction>().Where(searchPredicate).ToList();
}
public static S Add<C, S>(this S container, C element) where S : IAction where C : IInfo
{
container.Set<C, IAction>().Add(element);
return container;
}
public static C Remove<C>(this IAction container, Func<C, bool> searchFn) where C : IInfo
{
var obj = container.Set<C, IAction>().SingleOrDefault(searchFn);
if(obj != null)
{
container.Set<C, IAction>().Remove(obj);
}
return obj;
}
public static C AddInto<C>(this C obj, IAction container) where C : IInfo
{
container.Set<C, IAction>().Add(obj);
return obj;
}
public static void ForEach<T>(this IList<T> list, Action<T> action) where T : IInfo
{
for(int i = 0; i < list.Count; i++)
{
action(list[i]);
}
}
}
}
using System;
using System.Collections.Generic;
using System.Text;
namespace zestaw_6
{
public class Zoo : IInfo, IAction
{
public List<Klatka> klatki = new List<Klatka>();
public List<Opiekun> opiekunowie = new List<Opiekun>();
public void DisplayInfo()
{
foreach (var item in klatki)
{
item.DisplayInfo();
}
foreach (var item in opiekunowie)
{
item.DisplayInfo();
}
}
}
}
using System;
using System.Collections.Generic;
using System.Text;
namespace zestaw_6
{
public class Opiekun : IInfo, IAction
{
public List<Klatka> klatki = new List<Klatka>();
public string imie { get; set; }
public string nazwisko { get; set; }
public Opiekun(string imie_, string nazwisko_)
{
imie = imie_;
nazwisko = nazwisko_;
}
public void DisplayInfo()
{
Console.WriteLine("Imie: {0}", imie);
Console.WriteLine("Nazwisko: {0}", nazwisko);
foreach (var item in klatki)
{
item.DisplayInfo();
}
}
}
}
基本上,程序給出預期“System.NullReferenceException:“對象引用未設置為 object 的實例。” at Add function container.Set<C, IAction>().Add(element);. container.Set<C, IAction>() returns null and i dont know why. Funcion Add in main should Add new object to list "opiekunowie " 在 class 動物園里。那不起作用的原因是什么?
您的代碼使用反射,但沒有找到它想要的,有兩個原因。
反射在這里完成:
public static IList<TObj> Set<TObj, TAg>(this TAg aggregatedObj) where TObj : IInfo where TAg : IAction
{
var aggregatedObjType = aggregatedObj.GetType();
var propertyInfo = aggregatedObjType.GetProperties().FirstOrDefault(p => p.PropertyType == typeof(IList<TObj>));
var propertyValue = propertyInfo?.GetValue(aggregatedObj);
return propertyValue as IList<TObj>;
}
它查找IList<something>
類型的屬性(這是原因 1 )。
你在Zoo
里有:
public List<Klatka> klatki = new List<Klatka>();
public List<Opiekun> opiekunowie = new List<Opiekun>();
List<something>
類型的字段
像這樣正確(具有屬性):
public List<Klatka> klatki { get; set; } = new List<Klatka>();
public List<Opiekun> opiekunowie { get; set; } = new List<Opiekun>();
然后(檢查可分配給IList<something>
的類型的屬性(這是原因 2 )
var propertyInfo = aggregatedObjType.GetProperties().FirstOrDefault(p => typeof(IList<TObj>).IsAssignableFrom(p.PropertyType));
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