Add a try-catch with Mono Cecil

Adding exception handlers with Mono.cecil isn't difficult, it just requires you to know how exception handlers was laid O UT 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 so 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's, you simply want-wrap the existing code in a try catch handler. You can do it easily with Cecil the This is:

    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 was 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 Finall Y (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 are that the end instruction of a range are not contained inside the range. It ' s basically a [trystart:tryend[range.

Add a try-catch with Mono Cecil