Program.cs

using System.IO;
using System.Linq;
using Mono.Cecil;
using Mono.Cecil.Cil;

namespace NutstoreAutoPatch
{
    internal class Program
    {
        public static void Main(string[] args)
        {
            const string nutstoreLib = "NutstoreLib.dll";
            const string nutstoreLibBackup = "NutstoreLib.origin.dll";
            const string nutstoreLibTarget = "NutstoreLib.target.dll";
            
            const string directoryutils = "NutstoreLib.Utils.DirectoryUtils";
            const string appdataNutstoreDir = "get_APPDATA_NUTSTORE_DIR";

            // backup origin 
            File.Copy(nutstoreLib, nutstoreLibBackup, true);
            // Read the assembly
            using (var assemblyDefinition = AssemblyDefinition.ReadAssembly(nutstoreLib))
            {
                var directoryUtil = assemblyDefinition.MainModule.Types.First(t => t.FullName == directoryutils);
                var appdataDirMethod = directoryUtil.Methods.First(m => m.Name == appdataNutstoreDir);

                // 清除当前指令
                appdataDirMethod.Body.Instructions.Clear();

                // 获取IL处理器
                var ilProcessor = appdataDirMethod.Body.GetILProcessor();

                // 构造调用NutstoreLib.Utils.DirectoryUtils.get_NUTSTORE_INSTALL_DIR的指令
                var getInstallDirMethod = appdataDirMethod.Module.ImportReference(
                    appdataDirMethod.DeclaringType.Module.Types
                        .First(t => t.Name == "DirectoryUtils" && t.Namespace == "NutstoreLib.Utils")
                        .Methods.First(m => m.Name == "get_NUTSTORE_INSTALL_DIR")
                );
                ilProcessor.Append(ilProcessor.Create(OpCodes.Call, getInstallDirMethod));

                // 构造调用System.IO.DirectoryInfo构造函数的指令
                var directoryInfoConstructor = appdataDirMethod.Module.ImportReference(
                    typeof(System.IO.DirectoryInfo).GetConstructor(new[] { typeof(string) })
                );
                ilProcessor.Append(ilProcessor.Create(OpCodes.Newobj, directoryInfoConstructor));

                // 构造获取DirectoryInfo.Parent属性的调用指令
                var getParentMethod = appdataDirMethod.Module.ImportReference(
                    typeof(System.IO.DirectoryInfo).GetProperty("Parent").GetGetMethod()
                );
                ilProcessor.Append(ilProcessor.Create(OpCodes.Call, getParentMethod));

                // 构造调用System.IO.FileSystemInfo.FullName属性的指令
                var getFullNameMethod = appdataDirMethod.Module.ImportReference(
                    typeof(System.IO.FileSystemInfo).GetProperty("FullName").GetGetMethod()
                );
                ilProcessor.Append(ilProcessor.Create(OpCodes.Callvirt, getFullNameMethod));

                // 加载字符串"UserData"
                ilProcessor.Append(ilProcessor.Create(OpCodes.Ldstr, "UserData"));

                // 构造调用Pri.LongPath.Path.Combine的指令
                var pathCombineMethod = appdataDirMethod.Module.ImportReference(
                    typeof(System.IO.Path).GetMethod("Combine", new[] { typeof(string), typeof(string) })
                );
                ilProcessor.Append(ilProcessor.Create(OpCodes.Call, pathCombineMethod));

                // 返回指令
                ilProcessor.Append(ilProcessor.Create(OpCodes.Ret));

                // Write the modified assembly
                assemblyDefinition.Write(nutstoreLibTarget);
            }
            File.Copy(nutstoreLibTarget,nutstoreLib,true);
            File.Delete(nutstoreLibTarget);
        }
    }
}