Converting to Base 26 in One Based mode
You could slightly simplify the code as follows:


Reduce the number of variables.  
Use the LinkedList&lt;T&gt; (and the AddFirst method) instead of the List&lt;T&gt; to eliminate reversion of array.
Alternatively you can use the List&lt;T&gt;.Insert method to achieve the same result.


The code:

private static string ToBase26(int myNumber)
{
    var array = new LinkedList&lt;int&gt;();

    while (myNumber &gt; 26)
    {
        int value = myNumber % 26;
        if (value == 0)
        {
            myNumber = myNumber / 26 - 1;
            array.AddFirst(26);
        }
        else
        {
            myNumber /= 26;
            array.AddFirst(value);
        }
    }

    if (myNumber &gt; 0)
    {
        array.AddFirst(myNumber);
    }
    return new string(array.Select(s =&gt; (char)('A' + s - 1)).ToArray());
}