Question

I know some codes here are not clear. I’m still in the process of trying out things. I’ve got three questions, why does an error shows up when adding a string in my list? How do I pass the List in Class1 to my main class? And is my syntax correct in List passArr? Not sure if I should add parenthesis at the end of passArr.

class Class1
{
    public static List<string> passArr
    {
        get;
        set;
    }

    public static void passIt()
    {
        passArr.Add("A"); //Error: Object reference not set to an instance of an object
    }
}

Main Class

class Program
{
    static void Main(string[] args)
    {
        Class1.passIt();
        List<string> passArr1 = Class1.passArr;
        foreach (string s in passArr1)
        {
            Console.WriteLine(s);
        }
        Console.ReadLine();
    }
}

Answer 1

You’re never creating a list, so the passArr property (which needs renaming, by the way) always has a value of null. You need something like:

Class1.passArr = new List<string>();

at some point. Another alternative would be to make it a read-only property backed by a field with an initializer:

private static readonly List<string> passArr = new List<string>();
public static List<string> PassArr { get { return passArr; } }

(This complies with naming convention in terms of case, but it’s still not a meaningful name, of course.)