NullReferenceException 是列表和随机的未处理错误

Question

我有以下代码示例：

public int intTest { get; set; }
public List<int> listTest { get; set; }

//Method for creating a random number to return.
public int methodRandom()
{
    Random random1 = new Random();
    int intValue;
    intValue = random1.Next(1, intTest + 1);
    return (intValue);
}

private void button1_Click(object sender, EventArgs e)
{
    intTest = int.Parse(textBox1.Text); // Will be a value between 1 and 9 in my code)
    for (int i = 0; i < intTest; i++)
    {
        int temp = methodRandom();
        listTest.Add(temp);
    }
}

但是当我调试并单击按钮时，我收到以下错误消息，标记为“listTest.Add(temp);”说“NullReferenceException 未处理”。我做错了什么？

Answer 1

private void button1_Click(object sender, EventArgs e)
{
    listTest=new List<int>();
    intTest = int.Parse(textBox1.Text); // Will be a value between 1 and 9 in my code)
    for (int i = 0; i < intTest; i++)
    {
        int temp = methodRandom();

        listTest.Add(temp);
    }
}

Answer 2

您必须先实例化列表，然后才能向其中添加项目。你的构造函数是一个很好的地方。

public YourClass()  // I don't know what the name of your class is.
{
    listTest = new List<int>();
}

您可能也想在每次button1_Click 事件触发时清除列表（除非您的意图只是继续向列表中添加数字）。

private void button1_Click(object sender, EventArgs e)
{
    listTest.Clear();  // Clear previous items before adding new ones

    ...
}