Object reference not set to an instance of object when using a List<T> in C# [duplicate]

问题

This question already has answers here:

What is a NullReferenceException, and how do I fix it? (29 answers)

Closed 5 years ago.

I have the following code snippet that produces a compilation error:

public List<string> batchaddresses;

public MapFiles(string [] addresses)
{
    for (int i = 0; i < addresses.Count(); i++)
    {
        batchaddresses.AddRange(Directory.GetFiles(addresses[i], "*.esy"));
    }
}

I get an error when I try to use the List<T>.AddRange() method:

Object reference not set to an instance of an object

What am I doing wrong?

回答1:

Where is batchaddresses initialized?

Declaring the variable does not suffice. You must initialize it, like so:

// In your constructor
batchaddresses = new List<string>();

// Directly at declaration:
public List<string> batchaddresses = new List<string>();

回答2:

you have to initialize the list

List<String> batchaddresses = new List<String>();

回答3:

The batchaddresses field hasn't been initialised. You can initialise it as part of the declaration:

public List<string> batchaddresses = new List<string>();

回答4:

From your snippet, it doesn't look as though batchaddresses is initialised. Replace the line with this:

public List<string> batchaddresses = new List<string>();

来源：https://stackoverflow.com/questions/4466878/object-reference-not-set-to-an-instance-of-object-when-using-a-listt-in-c-shar