Auto Generate alphanumeric Unique Id with C#

Question

Total string length is 5 chars

I have a scenario, ID starts with

A0001 and ends with A9999 then B0001 to B9999 until F0001 to f9999

after that

Answer 1

Run this query in order to get the last ID in the database

SELECT TOP 1 [ID_COLUMN] FROM [NAME_OF_TABLE] ORDER BY [ID_COLUMN] DESC

Read the result to a variable and then run the following function on the result in order to get the next ID.

public string NextID(string lastID)
{
    var allLetters = new string[] {"A", "B", "C", "D", "E", "F"};
    var lastLetter = lastID.Substring(0, 1);
    var lastNumber = int.Parse(lastID.Substring(1));

    if (Array.IndexOf(allLetters, lastLetter) < allLetters.Length - 1 && 
        lastNumber == 9999) 
    {
        //increase the letter
        lastLetter = allLetters(Array.IndexOf(allLetters, lastLetter) + 1);
        lastNumber = 0;
    } else {
        lastLetter = "!";
    }

    var result = lastLetter + (lastNumber + 1).ToString("0000");

    //ensure we haven't exceeded the upper limit
    if (result.SubString(0, 1) == "!") {
        result = "Upper Bounds Exceeded!";
    }

    return result;
}

DISCLAIMER
This code will only generate the first set of IDs. I do not understand the process of generating the second set.