What happens if we add lines to IL code and add breakpoints to our program?

Question

If I add let's say 1 line at the beggining of a method, if I set a breakpoint through Visual Studio on the first line, will it point to the first line or the second? If it will flag the wrong line, is there anything we could do when editing .exe files to ensure a regular debugging session later?

Isn't there something like setting line x to be Y? I remember seeing something like that somewhere, not sure if .NET related or not.

Answer 1

You'll need to update the debugging symbols in the PDB file if you want the debugging experience to remain unchanged.

The best option for this I've seen is to use Mono.Cecil, as it supports (limited) modification of the debugging symbols as well as the IL.

Answer 2

If you are modifying IL then the PDB files will contain outdated information. Note that there probably won't be a 1:1 between changes in IL lines to the C# line #s (eg inserting 3 IL statements won't offset the IDE breakpoint by 3 lines of C#).

You may want to separate the IL-modified portions of your code into separate methods to minimize the impact. Also, assuming you are the one doing the IL modification, you may find it convenient to switch between the C# & IL views while debugging.

You might need to muck a bit with the generated code to facilitate that. For example, if the injected IL can be in a wrapper method then you could tell the debugger to ignore it by usage of attrbiutes such as DebuggerStepThroughAttribute, DebuggerNonUserCodeAttribute, or DebuggerHiddenAttribute. (Look up the documentation for the nuances in behavior)

Answer 3

I expect that you get flagged as .pdb file might not match.

Answer 4

How are you adding the IL? If you are doing this via profiler instrumentation (SetILFunctionBody) then you need to supply a new IL code map as well (SetILInstrumentationCodeMap) so that the debugger becomes aware of the IL modification.