I’m working on a key-generation system in C#. The logic should increment keys alphabetically, similar to how Excel columns work (e.g., A → B → C … Z → AA → AB). However, I’m experiencing unexpected behavior where keys reset after the first iteration in a loop.

I implemented two pieces of code for generating keys.

Code A (Works as Expected)

Key _key = new Key();

var a = _key.GenerateNextKey();
var b = a.GenerateNextKey();
var c = b.GenerateNextKey();
var d = c.GenerateNextKey();
var e = d.GenerateNextKey();
var f = e.GenerateNextKey();

Console.WriteLine(a);
Console.WriteLine(b);
Console.WriteLine(c);
Console.WriteLine(d);
Console.WriteLine(e);
Console.WriteLine(f);

This works as expected, generating keys sequentially from A to F.

Code B (Unexpected Behavior)

Key _key = new Key();

for (int i = 0; i < 5; i++)
{
    _key = _key.GenerateNextKey();
    Console.WriteLine(_key);
}

Expected Output:

B
C
D
E
F

Actual Output:

A
A
A
A
A

I expected Code B to increment keys just like Code A, but instead, the key resets to A after the first iteration.

What Did You Expect to Happen?

I expected the key to increment sequentially after every iteration of the loop, producing:

A
B
C
D
E
F

However, after the first iteration, the output resets to A repeatedly.

The Original GenerateNextKey Method

Here is the original GenerateNextKey method that I am using:

public Key GenerateNextKey()
{
    // Stack to keep track of the new characters
    Stack<char> newKey = new Stack<char>();
    Stack<char> currentCopy = new Stack<char>();

    // A flag to determine if a solution was found or not
    bool isIncremented = false;
    int lastAlphabetOccurences = 0;
    
    while (_keyChars.Count > 0)
    {
        // Pop the top element
        char currentTop = _keyChars.Pop();

        // Add it to the copy
        currentCopy.Push(currentTop);

        // Check if the current top element is the 'Z' element and the next key has not been generated
        if (currentTop != 'Z' && !isIncremented)
        {
            // Generate the key by incrementing the current top element
            currentTop++;
            isIncremented = true;
        }
        else if (currentTop == 'Z' && !isIncremented)
        {
            // Increment the top element to be an 'A'
            currentTop = 'A';

            // Count the 'Z' occurrences
            lastAlphabetOccurences++;
        }

        // Count the 'Z' occurrences
        if (currentTop == 'Z' && !isIncremented)
            lastAlphabetOccurences++;

        // Push to the newKey stack
        newKey.Push(currentTop);
    } // end while

    // Override the old stack with the new stack
    // Reverse the order of the copy stack
    _keyChars = currentCopy.ToReverseStack();

    // Check if we need to increment the string, i.e., we have to add 'A'
    if (_keyChars.Count == lastAlphabetOccurences)
        newKey.Push('A');

    return new Key(newKey.ToReverseStack());
}

Let me know if any further clarification is needed.