Question

How do I write a property for a list inside of a struct?

my code:

public struct Config
{
    List<int> ipAddress = new List<int>();
}

Answer 1

If all you want is to create an auto-property (which will default to null for reference types and cannot be initialized) you can do as @scartag suggests.

public struct Config
{
    // will default to null
    List<int> ipAddress {get; set;}
}

However, if you’re trying to do what’s in your code and initialize it to an actual reference, you will run into issues because you can’t initialize fields in a struct. They can only have their defaults. To make matters even worse, you can’t override a default constructor to do this for you.

Generally speaking, struct tends to be best for small, preferably immutable types. Is there a reason you don’t want to just use class for this?

Now, if you did want to create a struct with an “initialized” field, you can fool it with some lazy logic:

public struct Config
{
    private List<int> _ipAddress;
    private bool _isAssigned;

    public List<int> 
    {
        get 
        {
            if (!_isAssigned)
                _ipAddress = new List<int>;
            return _ipAddress; 
        }
        set
        {
            _ipAddress = value;
            _isAssigned = true;
        }
    }
}

But really, at this rate it’s better to use a class since you can initialize fields, or override the default constructor:

public class Config
{
    public List<int> ipAddress {get; set;}

    public Config()
    {
        ipAddress = new List<int>();
    }
}

And again, as Andrew mentioned in the comments, I strongly suggest looking at MSDN guidance on choosing between struct and class