How do i make a line (or lines) readonly.

I would like to make certain lines readonly.

Using this example:

1. using XYZ;
2. public class MyClass
3. {
4. public void MyMethod1()
5. {
6 do smth;
7. do smth else;
8. }
9. public void MyMethod2()
10. {
11. do smth;
12. do smth else;
13. do even more;
14. }
15. }

I would like to make lines 1-5 and 8-10 and 14-15 readonly, thus allowing the user to modify only the contents of one of the 2 C# methods. Line numbers will not work, because the user will be adding many lines within each method. I thought there may be something with SpanIndicators?

Please provide details.