程序中的System.NullReferenceException错误

Question

在我的程序中,我收到错误:

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添加内容时会发生这种情况,如下所示.TransactionList是一个类实例列表,声明如下:

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

这是Transaction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; }
}

有什么线索在这里错了吗？我似乎无法找到为什么抛出这个错误...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

只是在你之前初始化它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,如果你不喜欢它,既然你已经宣布它static,你可以这样做:

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