Adding static constructor with Mono.Cecil causes TypeInitializationException

Question

I am trying to add a static constructor using Mono Cecil to a program like the following:

namespace SimpleTarget
{
    class C
    {
        public void M()
        {
            Console.WriteLine("Hello, World!");
        }
    }
}

The following code adds the static constructor:

namespace AddStaticConstructor
{
    class Program
    {
        static void Main(string[] args)
        {
            var assemblyPath = args[0];
            var module = ModuleDefinition.ReadModule(assemblyPath);

            var corlib = ModuleDefinition.ReadModule(typeof(object).Module.FullyQualifiedName);
            var method = corlib.Types.First(t => t.Name.Equals("Console")).Methods.First(m => m.Name.Contains("WriteLine"));

            var methodToCall = module.Import(method);

            foreach (var type in module.Types)
            {
                if (!type.Name.Contains("C")) continue;

                var staticConstructorAttributes =
                    Mono.Cecil.MethodAttributes.Private |
                    Mono.Cecil.MethodAttributes.HideBySig |
                    Mono.Cecil.MethodAttributes.Static |
                    Mono.Cecil.MethodAttributes.SpecialName |
                    Mono.Cecil.MethodAttributes.RTSpecialName;

                MethodDefinition staticConstructor = new MethodDefinition(".cctor", staticConstructorAttributes, module.TypeSystem.Void);
                type.Methods.Add(staticConstructor);

                type.IsBeforeFieldInit = false;

                var il = staticConstructor.Body.GetILProcessor();
                il.Append(Instruction.Create(OpCodes.Ret));

                Instruction ldMethodName = il.Create(OpCodes.Ldstr, type.FullName);
                Instruction callOurMethod = il.Create(OpCodes.Call, methodToCall);

                Instruction firstInstruction = staticConstructor.Body.Instructions[0];
                // Inserts the callOurMethod instruction before the first instruction


                il.InsertBefore(firstInstruction, ldMethodName);
                il.InsertAfter(ldMethodName, callOurMethod);
            }

            module.Write(assemblyPath);
        }
    }
}

Looking at the decompiled binary in dotPeek, it appears as if everything is setup correctly. When trying to use the modified C type, I get a TypeInitializationException with the inner exception "System.InvalidProgramException: JIT Compiler encountered an internal limitation"

Is there anything else I need to set correctly before using a static constructor?

Thanks!

Answer 1

The problem is that you are getting the wrong overload of System.WriteLine here:

var corlib = ModuleDefinition.ReadModule(typeof(object).Module.FullyQualifiedName);
var method = corlib.Types.First(t => t.Name.Equals("Console")).Methods.First(m => m.Name.Contains("WriteLine"));
var methodToCall = module.Import(method);

use this simple code the get the overload you want to use:

var wlMethod = typeof (Console).GetMethod(nameof(Console.WriteLine), new[] {typeof (string)});
var methodToCall = module.ImportReference(wlMethod);