Add MaxCharactersAllowed property to TextBox same as MaxLength

Question

Hi,

In my project I set max char allowed for TextBox  using following

 TextBox.MaxLength = 20;
 
 I want add one more property as MaxCharactersAllowed. This will work same as MaxLength
 
 TextBox.MaxCharactersAllowed = 20;

I found that TextBox properties written in "System.Windows.Controls.TextBox.cs"
 
Any idea how to add property?

- Thanks