problems with null and empty string array

Question

class ToDoList { 
    public static void toDO() {

        List<string[]> addEntry = [];
        bool toDoLoop = true;

        while (toDoLoop) {
            Clear(); 
            WriteLine("\n\t\tHere is your to do list!");

            for (int i = 1; i < addEntry.Count; i++)
            {
                WriteLine($"\n\t\tTo Do {i}: {addEntry[i][0]}");
            }
            Write("\n\t\t[N]ew Entry\t[E]xit\t");

            string? userChoice = ReadLine();
            switch (userChoice)
            {
                case "n":
                case "N":
                    Write("\n\t\tAdd New Entry: ");
                    string?[] newToDo = new string[1];

                    newToDo[0] = ReadLine();

                    if (newToDo.Length == 0) { 
                        WriteLine("\n\t\tEntries can't be null!"); 
                    }

                    else { 
                        addEntry.Add(newToDo); 
                    }
                    break;

                case "e":
                case "E":
                    Clear();
                    WriteLine("\n\n\t\tClosing To Do!");
                    toDoLoop = false;
                    break;

                default:
                    Clear();
                    WriteLine("\n\t\tInvalid Entry!");
                    break;
            }
        }
    }
}

I have tried a different variants with having the add entry inside both the if and the else, but i don't get it to work. Am i thinking completely wrong or am i at least on the right path?

if (newToDo[0] == "")

if (newToDo[0] != )

if (newToDo[0] is null)

if (newToDo[0] is not null)

if (!string.IsNullOrEmpty(newToDo[0]))

Since you are apparently dealing with "entries" and "Todos" create classes that model an "Entry" and a "Todo". That should result in code much easier to read than using multiple levels of nested collections. — JonasH
– JonasH, Commented Jan 4, 2024 at 14:59

Joel Coehoorn · Accepted Answer · 2024-01-04 16:13:04Z

Arrays always have a fixed length. Anything that changes this is actually a new array. So when you have this line:

string?[] newToDo = new string[1];

we can know that newToDo.Length will always be 1, whether or not you've ever set a value for any of the items, unless you assign a whole new array to the reference. And therefore this check:

if (newToDo.Length == 0) {

will never be true. We already know we set it to an array with a .Length property of 1.

So which of the alternatives should you use? In my experience, the last option is most correct:

if (!string.IsNullOrEmpty(newToDo[0]))

but sometimes this is also useful:

if (!string.IsNullOrWhiteSpace(newToDo[0]))

But one wonders why you have a list of string[] at all, instead of a list of string (no array), which would be much simpler and still seems to meet your needs here.

While I'm here, indexer ([]) properties in C# start at 0, not 1, so this code is suspect:

for (int i = 1; i < addEntry.Count; i++)

You may want something more like this:

for (int i = 0; i < addEntry.Count; i++)
{
    WriteLine($"\n\t\tTo Do {i++}: {addEntry[i][0]}");
}

And then also make a corresponding -1 adjustment when reading an input of which line number to use.

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