How to read all lines inside Custom methods in C#

Well, you'll need to have access to the original .cs file.

This is because it's impossible to recreate the exact C# source code from a method's IL in a compiled assembly because comments are removed altogether and the original local variable names are replaced by numbered storage locations.

Subject to that you'd need to parse the .cs file to find the body of the method you want and isolate the source code it contains. You might be able to do this using regular expressions, rather than a dedicated C# parser, but it won't be particularly easy.