source: binary-improvements/assembly-patcher/Main.cs@ 191

using System;
using System.Collections.Generic;
using System.Reflection;
using Mono.Cecil;
using Mono.Cecil.Cil;

namespace dtdfixer
{
        class MainClass
        {
                public static void Main (string[] args)
                {
                        ModuleDefinition module = ModuleDefinition.ReadModule ("Assembly-CSharp.dll");

                        TypeDefinition type = module.GetType ("GameManager");
                        if (isPatched (type)) {
                                Console.WriteLine ("Assembly already patched");
                                return;
                        }
                        markTypePatched (module, type);

                        mappingPatch (module);
                        consoleOutputPatch (module);
                        telnetPatch (module);
                        connectLogPatch (module);
                        publicCommandPermissionsPatch (module);
                        playerDataPatch (module);

                        module.Write ("Assembly-CSharp.patched.dll");
                        Console.WriteLine ("Done");

                }

                private static void mappingPatch (ModuleDefinition module)
                {
                        TypeDefinition type = module.GetType ("Chunk");
                        addHook (type, "CalcMapColors", true, 0, true, typeof(AllocsFixes.MapRendering.MapRendering).GetMethod ("RenderSingleChunk"));
                }

                private static void consoleOutputPatch (ModuleDefinition module)
                {
                        TypeDefinition type = module.GetType ("ConsoleSdtd");
                        replaceMethod (type, "ExecuteCmdFromClient", true, 3, typeof(AllocsFixes.NetConnections.ConsoleOutputSeparator).GetMethod ("C_ExecuteCmdFromClient"));
                        addHook (type, "Run", true, 0, true, typeof(AllocsFixes.NetConnections.ConsoleOutputSeparator).GetMethod ("C_Run"));
                        replaceMethod (type, "SendResult", true, 1, typeof(AllocsFixes.NetConnections.ConsoleOutputSeparator).GetMethod ("C_SendResult"));
                }

                private static void playerDataPatch (ModuleDefinition module)
                {
                        TypeDefinition type = module.GetType ("GameManager");
                        addHook (type, "SavePlayerData", true, 2, true, typeof(AllocsFixes.PlayerDataStuff).GetMethod ("GM_SavePlayerData"));
                        addHook (type, "Awake", true, 0, true, typeof(AllocsFixes.StateManager).GetMethod ("Awake"));
                        addHook (type, "Shutdown", true, 0, false, typeof(AllocsFixes.StateManager).GetMethod ("Shutdown"));
                }

                private static void publicCommandPermissionsPatch (ModuleDefinition module)
                {
                        TypeDefinition type = module.GetType ("AdminTools");
                        replaceMethod (type, "GetAllowedCommandsList", true, 1, typeof(AllocsFixes.AdminToolsStuff).GetMethod ("GetAllowedCommandsList"));
                }

                private static void connectLogPatch (ModuleDefinition module)
                {
                        TypeDefinition type = module.GetType ("GameManager");
                        addHook (type, "RequestToSpawnPlayer", true, 5, true, typeof(AllocsFixes.AllocsLogFunctions).GetMethod ("RequestToSpawnPlayer"));
                        type = module.GetType ("ConnectionManager");
                        addHook (type, "RemovePlayer", true, 2, false, typeof(AllocsFixes.AllocsLogFunctions).GetMethod ("PlayerDisconnected"));
                }

                private static void telnetPatch (ModuleDefinition module)
                {
                        TypeDefinition type = module.GetType ("NetTelnetServer");
                        replaceMethod (type, ".ctor", false, 1, typeof(AllocsFixes.NetConnections.NetTelnetServer).GetMethod ("init"));
                        replaceMethod (type, "Disconnect", false, 0, typeof(AllocsFixes.NetConnections.NetTelnetServer).GetMethod ("Disconnect"));
                        replaceMethod (type, "SetConsole", false, 1, typeof(AllocsFixes.NetConnections.NetTelnetServer).GetMethod ("SetConsole"));
                        replaceMethod (type, "WriteToClient", false, 1, typeof(AllocsFixes.NetConnections.NetTelnetServer).GetMethod ("WriteToClient"));
                }

                private static void markTypePatched (ModuleDefinition module, TypeDefinition type)
                {
                        type.Fields.Add (new FieldDefinition ("AllocsPatch", Mono.Cecil.FieldAttributes.Private | Mono.Cecil.FieldAttributes.SpecialName, module.Import (typeof(int))));
                }

                private static void addHook (TypeDefinition type, string methodName, bool addThisRef, int opCount, bool atEnd, MethodBase targetMethod)
                {
                        foreach (MethodDefinition method in type.Methods) {
                                if (method.Name.Equals (methodName)) {
                                        var il = method.Body.GetILProcessor ();
                                        var call = il.Create (OpCodes.Call, method.Module.Import (targetMethod));
                                        if (atEnd) {
                                                int insBefore = method.Body.Instructions.Count;
                                                if (addThisRef)
                                                        il.Append (il.Create (OpCodes.Ldarg, 0));
                                                for (int op = 0; op < opCount; op++) {
                                                        il.Append (il.Create (OpCodes.Ldarg, op + 1));
                                                }
                                                il.Append (call);
                                                il.Remove (method.Body.Instructions [insBefore - 1]);
                                                il.Append (il.Create (OpCodes.Ret));
                                        } else {
                                                var i = 0;
                                                if (addThisRef)
                                                        il.InsertBefore (method.Body.Instructions [i++], il.Create (OpCodes.Ldarg, 0));
                                                for (int op = 0; op < opCount; op++) {
                                                        il.InsertBefore (method.Body.Instructions [i++], il.Create (OpCodes.Ldarg, op + 1));
                                                }
                                                il.InsertBefore (method.Body.Instructions [i++], call);
                                        }
                                        return;
                                }
                        }
                        Console.WriteLine ("ERROR: Did not find " + type.Name + "." + methodName + "()");
                }

                private static void replaceMethod (TypeDefinition type, string methodName, bool addThisRef, int opCount, MethodBase targetMethod)
                {
                        foreach (MethodDefinition method in type.Methods) {
                                if (method.Name.Equals (methodName)) {
                                        var il = method.Body.GetILProcessor ();
                                        var call = il.Create (OpCodes.Call, method.Module.Import (targetMethod));
                                        var i = 0;
                                        if (addThisRef)
                                                il.InsertBefore (method.Body.Instructions [i++], il.Create (OpCodes.Ldarg, 0));
                                        for (int op = 0; op < opCount; op++) {
                                                il.InsertBefore (method.Body.Instructions [i++], il.Create (OpCodes.Ldarg, op + 1));
                                        }
                                        il.InsertBefore (method.Body.Instructions [i++], call);
                                        il.InsertBefore (method.Body.Instructions [i++], il.Create (OpCodes.Ret));
                                        return;
                                }
                        }
                        Console.WriteLine ("ERROR: Did not find " + type.Name + "." + methodName + "()");
                }

                private static bool isPatched (TypeDefinition type)
                {
                        foreach (FieldDefinition fd in type.Fields) {
                                if (fd.Name.Equals ("AllocsPatch")) {
                                        return true;
                                }
                        }
                        return false;
                }
        }
}