I want to write a simple propety for textbox.

I know a structure of property statement but i cant do exactly.

example :

I want create a property name selectedlength1. And i have two textboxs in my form and i want to assign :

Textbox1.Selectedlength1 = 2
Textbox1.Selectedlength1 = 3

Then the length of string in textbox1 is selected is : 2 and 3 for textbox2.

How can i do that. ?