c# - 程序中的 System.NullReferenceException 错误

Question

在我的程序中，我收到了错误：

An unhandled exception of type 'System.NullReferenceException' occurred in POS    System.exe

Additional information: Object reference not set to an instance of an object.

当我尝试向 TransactionList 添加一些内容时会发生这种情况，如下所示。TransactionList 是类实例的列表，声明如下：

public static List<Transaction> TransactionList { get; set; }

这是事务类：

class Transaction
{
    public double TotalEarned { get; set; }
    public double TotalHST { get; set; }
    public double TotalCost { get; set; }
    public string Category { get; set; }
    public int DaysSince2013 { get; set; }
}

任何线索这里有什么问题？我似乎无法找到引发此错误的原因...谢谢！

for (int i = 0; i < (lines / 5); i++)
        {
            TransactionList.Add(new Transaction //Error happens on this line
            {
                TotalEarned = Convert.ToDouble(stringArray[(i * 5)]),
                TotalCost = Convert.ToDouble(stringArray[(i * 5) + 1]),
                TotalHST = Convert.ToDouble(stringArray[(i * 5) + 2]),
                Category = stringArray[(i * 5) + 3],
                DaysSince2013 = Convert.ToInt32(stringArray[(i * 5) + 4])
            });
        }

Answer 1

只需在您的for loop?

if (TransactionList == null)
   TransactionList = new List<Transaction>();

for (int i = 0; i < (lines / 5); i++)
        {
            TransactionList.Add(new Transaction //Error happens on this line
            {
                TotalEarned = Convert.ToDouble(stringArray[(i * 5)]),
                TotalCost = Convert.ToDouble(stringArray[(i * 5) + 1]),
                TotalHST = Convert.ToDouble(stringArray[(i * 5) + 2]),
                Category = stringArray[(i * 5) + 3],
                DaysSince2013 = Convert.ToInt32(stringArray[(i * 5) + 4])
            });
        }

或者，如果您不喜欢这样，因为您已将其声明为static，您可以这样做：

public static List<Transaction> TransactionList = new List<TransactionList>();

Answer 2

您必须在使用它之前初始化列表。当您得到 Object reference not set to an instnce 错误时，这意味着该对象物理上不存在

Answer 3

可能 TransactionList 和/或 stringArray 都是空的。

尝试这样做

公共静态列表 TransactionList { 获取；放; }

if(TransactionList  == null)
   TransactionList  = new List<Transaction>();