When trying to change an icon in a TextBox using the Segoe MDL2 Assets, it just give me some ugly squares.
The solution is to use the escape character followed by the Unicode representation of the icon.
MyTextBox.Content = "\uE71A";
And not to use the classical notation &#…
Thanks to this guy : https://www.youtube.com/watch?v=SOkOjjEQzxc