Add a try-catch with Mono Cecil

Adding exception handlers with Mono.Cecil is not difficult, it just requires you to know how exception handlers are laid out in the metadata.

Let say you have the C# method:

static void Throw ()
{
    throw new Exception ("oups");
}

If you decompile it, it should look somewhat similar to this:

.method private static hidebysig default void Throw ()  cil managed
{
    IL_0000:  ldstr "oups"
    IL_0005:  newobj instance void class [mscorlib]System.Exception::.ctor(string)
    IL_000a:  throw
}

Now let say that you want to inject code in this method such as it‘s similar to the C# code:

static void Throw ()
{
    try {
        throw new Exception ("oups");
    } catch (Exception e) {
        Console.WriteLine (e);
    }
}

That is, you simply want to wrap the existing code in a try catch handler. You can do it easily with Cecil this way:

    var method = ...;
    var il = method.Body.GetILProcessor ();

    var write = il.Create (
        OpCodes.Call,
        module.Import (typeof (Console).GetMethod ("WriteLine", new [] { typeof (object)})));
    var ret = il.Create (OpCodes.Ret);
    var leave = il.Create (OpCodes.Leave, ret);

    il.InsertAfter (
        method.Body.Instructions.Last (),
        write);

    il.InsertAfter (write, leave);
    il.InsertAfter (leave, ret);

    var handler = new ExceptionHandler (ExceptionHandlerType.Catch) {
        TryStart = method.Body.Instructions.First (),
        TryEnd = write,
        HandlerStart = write,
        HandlerEnd = ret,
        CatchType = module.Import (typeof (Exception)),
    };

    method.Body.ExceptionHandlers.Add (handler);

This code is manipulating the previous method to look like this:

.method private static hidebysig default void Throw ()  cil managed
{
    .maxstack 1
    .try { // 0
      IL_0000:  ldstr "oups"
      IL_0005:  newobj instance void class [mscorlib]System.Exception::‘.ctor‘(string)
      IL_000a:  throw
    } // end .try 0
    catch class [mscorlib]System.Exception { // 0
      IL_000b:  call void class [mscorlib]System.Console::WriteLine(object)
      IL_0010:  leave IL_0015
    } // end handler 0
    IL_0015:  ret
}

We‘re adding three new instructions: a call to Console.WriteLine, a leave to gracefully exit the catch handler, and finally (pun intended), a ret. Then we‘re simply creating a ExceptionHandler instance to represent a try catch handler whose try encompasses the existing body, and whose catch is the WriteLine statement.

One important thing to note is that the end instruction of a range is not contained inside the range. It‘s basically a [TryStart:TryEnd[ range.