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

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

public List 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.AddRange() method:

Object reference not set to an instance of an object

What am I doing wrong?

Answer

Where is batchaddresses initialized?

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

// In your constructor
batchaddresses = new List();

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