how can i modify memory loaded methods in given assembly so that clr when instantiates new objects for some class it makes new objects with modified code in it?
You can use Mono Cecil to modify the assembly before you load it.
You can save the modified assembly to a byte array, then call Assembly.Load to load the byte array.
I don't think you can do that. But you can generate new classes in a new, in-memory assembly using Reflection.Emit.
That is a lot of work though. You can get help from a framework such as Castle DynamicProxy, which allows you to generate proxies for your classes at runtime. This way you can run any code before or after the original methods run, or change the methods totally by not calling into the original method.
来源：https://stackoverflow.com/questions/3008192/modify-the-assembly-on-runtime