Object reference not set to an instance of object when using a List in C#

Question

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?

Answer 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>();

Answer 2

you have to initialize the list

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

Answer 3

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

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

Answer 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>();