我该如何处理这个NullReferenceException？

Question

public partial class Form1 : Form
{
    string[] id;

private void button_Click(object sender, EventArgs e)
{
    char[] delimiters = { ',', '\r', '\n' };
    string[] content = File.ReadAllText(CSV_File).Split(delimiters);

    int x = content.GetUpperBounds(0)
    int z = 0;
    int i - 0;

    for (i = 0; i <= x / 3; i++)
        {
            z = (i * 3);
            id[i] = content[z]; // this line gives the error
        }

}
}

I want to get every 3rd value from array content, and put it into array id. This gives a 'NullReferenceException was unhandled' error and suggests I use 'new', but it is not a type or namespace. What should I do here?

They are both string arrays, and the error occurs on the first run so I do not think it is related to exceeding the bounds.

Answer 1

您需要在for循环之前初始化id数组：

id = new string[x/3];

Answer 2

This line of code:

string[] id;

is actually creating a null reference.

When you declare an array, you have to explicitly create it, specifying the size.

In your example, you have two alternatives

1. Determine how big the array will be beforehand, and create the array length
2. Actually populate a container that manages its own size.

The first option:

int x = content.GetUpperBounds(0)
int z = 0;
int i - 0;

id = new string[x/3];
for (i = 0; i <= x / 3; i++)
    {
        z = (i * 3);
        id[i] = content[x];
    }

The second option:

int x = content.GetUpperBounds(0)
int z = 0;
int i - 0;

List<string> list = new List<string>();
for (i = 0; i <= x / 3; i++)
    {
        z = (i * 3);
        list.Add(content[z]);
    }

id = list.ToArray();

The first option would perform better, as you are only allocating one object.

Admittedly, I tend to disregard performance and use the second option, because it takes less brainpower to code.