Select the next TextBox in order
You absolutely can use a Dictionary solution.  Here is what it would look like.

In some initialization code, maybe the form's constructor, write the following code:

// global mapping
private Dictionary&lt;TextBox, TextBox&gt; nextTextboxMap = new Dictionary&lt;TextBox, TextBox&gt;();

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.
}


And then, your SelectNextTextBox() method can be simplified to this:

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;
    }
}