private void SelectNextTextBox()
    {
        TextBox newTextBox = null;

        if (_currentTextBox == keyText1)
        {
            newTextBox = keyText2;
        }
        else if (this._currentTextBox == keyText2)
        {
            newTextBox = keyText3;
        }
        else if (this._currentTextBox == keyText3)
        {
            newTextBox = keyText4;
        }
        else if (this._currentTextBox == keyText4)
        {
            newTextBox = keyText5;
        }
        else if (this._currentTextBox == this.keyText5)
        {
            newTextBox = regText1;
        }
        else if (this._currentTextBox == regText1)
        {
            newTextBox = regText2;
        }
        else if (this._currentTextBox == regText2)
        {
            newTextBox = regText3;
        }
        else if (this._currentTextBox == regText3)
        {
            newTextBox = regText4;
        }
        else if (this._currentTextBox == regText4)
        {
            newTextBox = regText5;
        }
        else
        {
            return;
        }
        newTextBox.SelectAll();
        newTextBox.Focus();
    }

// global mapping
private Dictionary<TextBox, TextBox> nextTextboxMap = new Dictionary<TextBox, TextBox>();

private void InitNextTextBoxMap()
{
    this.nextTextboxMap[keyText1] = keyText2;
    this.nextTextboxMap[keyText2] = keyText3;
    this.nextTextboxMap[keyText3] = keyText4;
    // add the other mappings here
    // where key = current textbox, and value = next textbox.
}

private void SelectNextTextBox()
{
    TextBox newTextBox = null;
    if (this.nextTextboxMap.TryGetValue(this._currentTextBox, out newTextBox))
    {
        newTextBox.SelectAll();
        newTextBox.Focus();

        // Maybe you also want to reset this._currentTextBox at this point?
        // this._currentTextBox = newTextBox;
    }
}

TextBox[] allTextBoxes = new TextBox[]{
    keyText1,
    keyText2,
    keyText3,
    keyText4,
    keyText5,
    regText1,
    regText2,
    regText3
};

int currentIndex = Array.IndexOf(allTextBoxes, this._currentTextBox);

if(currentIndex > 0)
{
    int nextIndex = (currentIndex + 1) % allTextBoxes.Length;
    TextBox nextTextBox = allTextBoxes[nextIndex];
    nextTextBox.SelectAll();
    nextTextBox.Focus();
}

List<TextBox> boxes;
int boxesIndex;

boxes = new List<TextBox>(); // Then add each text box to list from first to last.
boxesIndex = -1;

if(boxes == null || boxes.Count == 0)
    throw new Exception("No textboxes have been set");

Textbox tb;
if(++boxesIndex >= boxes.Count)
{
    boxesIndex = 0;
}

tb = boxes[boxesIndex];
tb.SelectAll();
tb.Focus();