Starting text is disappeared if the rich textbox has no scroll bar

[C# Programming]

Hi,

I am using c#.net to create a windows application
in that i have a rich text box control
when ever the text comes to end in the rich text box
for every new line in that rich text box the first line becomes disappears

now what i need to do is i have to restrict the user
when ever the rich text box becomes full

please advice me how can i do this

Thanks
Anil Veeraghattapu