Programmatic MSIL injection

Question

Let's say I have a buggy application like this:

using System;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("2 + 1 = {0}", Add(2, 1));
        }

        static int Add(int x, int y)
        {
            return x + x; // <-- oops!
        }
    }
}

The application is already compiled and deployed into the wild. Someone has found the bug, and now they are requesting a fix for it. While I could re-deploy this application with the fix, its an extreme hassle for reasons outside of my control -- I just want to write a patch for the bug instead.

Specifically, I want to insert my own MSIL into the offending assmembly source file. I've never done anything like this before, and googling hasn't turned up any useful information. If I could just see a sample of how to do this on the code above, it would help me out tremendously :)

How do I programmatically inject my own MSIL into a compiled .NET assembly?

[Edit to add:] To those who asked: I don't need runtime hotswapping. Its perfectly fine for me to have the app closed, manipulate the assembly, then restart the program again.

[Edit one more time:] It looks like the general consensus is "manipulating the assembly is a bad way to patch a program". I won't go down that road if its a bad idea.

I'll leave the question open because MSIL injection might still be useful for other purposes :)

Answer 1

I guess the question I would ask is "how would you deploy the patch"? Somewhere, you have to deploy something to fix a bug that is already out in the wild. Why would recompiling the dll and releasing the fixed version really be an issue? My guess is that figuring out how to programatically inject MSIL is going to be more trouble than simply redeploying a fixed assembly.

Answer 2

Rather than injecting MSIL at runtime, have you considered inserting the source directly into the assembly?

You can disassemble with ildasm, insert your MSIL, and then reassemble with ilasm, then deploy the product of that.