Generating a sequence number based on last item of a list

Question

I wrote this method in order to generate a sequence number based on a specific pattern:

public string LatestNumber(Guid id = default(Guid))
{
    var number = "";
    if (id == Guid.Empty)
    {
        var items = myList.ToList();
        var lastNumber = items.Any() ? items[items.Count - 1].Number : "";

        var intitialNumber = "Number#000001";
        if (!items.Any())
            number = intitialNumber;
        else
        {
            var nextNumber = int.Parse(lastNumber.Split('#')[1]) + 1;
            number = $"Number#{nextNumber:D6}";
        }
    }
    else
    {
        number = myList.FirstOrDefault(p => p.Id == id).Number;
    }

    return number;
}

This code works as expected in production but I wonder if there's a better way to make this code clean or more testable. What do you think?

Answer 1

There are a couple of things you could do differently.

• number - this variable is not necessary, you can return anytime
• myList - if this is already a list then you don't need to copy it
• lastNumber - this is initialized too early. It should be placed inside else
• if - don't ommit the {}
• !items.Any() and items.Any() - you don't need to check it twice and even with different results
• : "" - this value is never used because you don't enter the first else if myList.Any() - in this case you use .Number and the only place where you use the resulting lastNumber is the first else
• [1] - you should name this index
• intitialNumber - define this as a constant

---

number = myList.FirstOrDefault(p => p.Id == id).Number;

You know that this might blow? If the list is empty then the Number cannot be read. You need either C# 6 for this or one more line of code.

---

Putting everything together gives:

const string intitialNumber = "Number#000001";

if (id == Guid.Empty)
{
    if(myList.Any())
    {
        const int someMagicValue = 1;
        var lastNumber = myList.Last().Number; // Last is O(1) for IList
        var nextNumber = int.Parse(lastNumber.Split('#')[someMagicValue]) + 1;
        return = $"Number#{nextNumber:D6}";
    }
    else
    {
        return intitialNumber;
    }
}
else
{
    var first = myList.FirstOrDefault(p => p.Id == id);
    return first != null ? first.Number : intitialNumber;

    // C# 6
    // myList.FirstOrDefault(p => p.Id == id)?.Number ?? intitialNumber;
}