如何解决错误“对象引用未设置为对象的实例。” [复制]答案

【问题标题】：How can I resolve the error 'Object reference not set to an instance of an object.' [duplicate]如何解决错误“对象引用未设置为对象的实例。” [复制]
【发布时间】：2020-11-05 14:02:02
【问题描述】：

这是我的代码

 public class Bill 
{
   public int BillNumber { get; set; }
   public DateTime BillDate { get; set; }
   public List<BillLine> LineItems { get; set; }



 public void AddBillLine(BillLine billLine)
 {
     LineItems.Add(billLine);
 }

 public void RemoveBillLine(int SOMEID)
 {
     throw new NotImplementedException();
 }

 /// GetTotal returns the sum of (Cost * Quantity) for each line item

 public decimal GetTotal()
 {
     throw new NotImplementedException();
 }

 public void MergeBill(Bill sourceBill)
 {
     throw new NotImplementedException();
 }

 /// deep clone of the current bill (all fields and properties)

 public Bill Clone()
 {
     throw new NotImplementedException();
 }

 public override string ToString()
 {
     throw new NotImplementedException();
 }
}



  public class BillLine
{
 public int BillLineId { get; set; }
 public string Description { get; set; }
 public int Quantity { get; set; }
 public double Cost { get; set; }
 }




   public class Program
 {
    static void Main(string[] args)
 {
     Console.WriteLine("Billing app started....");

     CreateBillWithOneItem();
     CreateBillWithMultipleItemsAndQuantities();
     RemoveItem();
     MergeBill();
     CloneBill();
     BillToString();
 }

 private static void CreateBillWithOneItem()
 {
     var bill = new Bill();

     bill.AddBillLine(new BillLine()
     {
         BillLineId = 1,
         Cost = 6.99,
         Quantity = 1,
         Description = "Apple"
     });

     Console.WriteLine(bill.GetTotal());
 }

 private static void CreateBillWithMultipleItemsAndQuantities()
 {
     var bill = new Bill();

     bill.AddBillLine(new BillLine()
     {
         BillLineId = 1,
         Cost = 10.21,
         Quantity = 4,
         Description = "Banana"
     });

     bill.AddBillLine(new BillLine()
     {
         BillLineId = 2,
         Cost = 5.21,
         Quantity = 1,
         Description = "Orange"
     });

     bill.AddBillLine(new BillLine()
     {
         BillLineId = 3,
         Cost = 5.21,
         Quantity = 5,
         Description = "Pineapple"
     });

     Console.WriteLine(bill.GetTotal());
 }

 private static void RemoveItem()
 {
     var bill = new Bill();

     bill.AddBillLine(new BillLine()
     {
         BillLineId = 1,
         Cost = 5.21,
         Quantity = 1,
         Description = "Orange"
     });

     bill.AddBillLine(new BillLine()
     {
         BillLineId = 2,
         Cost = 10.99,
         Quantity = 4,
         Description = "Banana"
     });

     bill.RemoveBillLine(1);
     Console.WriteLine(bill.GetTotal());
 }

 private static void MergeBill()
 {
     var bill1 = new Bill();

     bill1.AddBillLine(new BillLine()
     {
         BillLineId = 1,
         Cost = 10.33,
         Quantity = 4,
         Description = "Banana"
     });

     var bill2 = new Bill();

     bill2.AddBillLine(new BillLine()
     {
         BillLineId = 2,
         Cost = 5.22,
         Quantity = 1,
         Description = "Orange"
     });

     bill2.AddBillLine(new BillLine()
     {
         BillLineId = 3,
         Cost = 6.27,
         Quantity = 3,
         Description = "Blueberries"
     });

     bill1.MergeBill(bill2);
     Console.WriteLine(bill1.GetTotal());
 }

 private static void CloneBill()
 {
     var bill = new Bill();

     bill.AddBillLine(new BillLine()
     {
         BillLineId = 1,
         Cost = 6.99,
         Quantity = 1,
         Description = "Apple"
     });

     bill.AddBillLine(new BillLine()
     {
         BillLineId = 2,
         Cost = 6.27,
         Quantity = 3,
         Description = "Blueberries"
     });

     var clonedBill = bill.Clone();
     Console.WriteLine(clonedBill.GetTotal());
 }

 private static void BillToString()
 {
     var bill = new Bill()
     {
         BillDate = DateTime.Now,
         BillNumber = 1000,
         LineItems = new List<BillLine>()
         {
             new BillLine()
             {
                 BillLineId = 1,
                 Cost = 6.99,
                 Quantity = 1,
                 Description = "Apple"
             }
         }
     };

     Console.WriteLine(bill.ToString());
 }

}

谁能建议如何解决这个错误：System.NullReferenceException: 'Object reference not set to an instance of an object.'
任何建议我需要在我的应用程序中做哪些必要的更改以避免错误
错误来自 LineItems.Add(billLine);

例如。尝试添加

List<BillLineItem> BillLineItems = new List<BillLineItem>();

public void AddBillLine(BillLine billLine)
    {
        LineItems.Add(billLine);
    }

【问题讨论】：

LineItems 空。你需要有List<BillLineItem> LineItems = new List<BillLineItem>();

标签： c# .net .net-core c#-4.0 console-application

【解决方案1】：

您收到null reference 异常的原因是Bill 类中的这一行：

public List<BillLine> LineItems { get; set; }

目前没有初始化列表。最简单的解决方法是：

public List<BillLine> LineItems { get; set; } = new List<BillLine>();

【讨论】：

【解决方案2】：

在程序开始时，您调用方法“CreateBillWithOneItem”。然后，此方法创建一个新的 Bill 对象并调用方法“AddBillLine”。 'AddBillLine' 接受变量 LineItems 并尝试添加一个新的 BillLine 对象。

到目前为止，您还没有通过将 LineItems 设置为对象来实例化它。因此，到目前为止它是空的。尝试在空对象上调用方法时，您将收到该错误。

您确实在 'BillToString' 中通过将其设置为该类型的新对象来实例化它，但这会在程序中稍后发生。

【讨论】：

【解决方案3】：

除了@Hayden 所说的，您还可以为初始化 LineItems 的 Bill 类创建一个构造函数。由于您没有在 Bill 中指定构造函数，因此您调用的是默认构造函数，

var bill = new Bill();

尝试将构造函数添加到初始化 LineItems 的 Bill 类，

public Bill()
{
    LineItems = new List<BillLine>();
}

【讨论】：