Как получить значение реестра с удаленного компьютера, если я не знаю его тип? (С#)

можно использовать StdRegProv.EnumValues метод для перечисления значений ключа, узнать тип вашего значения и определить, который Get метод для вызова.

что-то вроде этого (извините, я не знаю C# хорошо, поэтому этот код, вероятно, далек от совершенства):

    using System;
    using System.Management;
    using System.Management.Instrumentation;

    namespace ConsoleApplication1
    {
        public enum RegHive : uint
        {
            HKEY_CLASSES_ROOT = 0x80000000,
            HKEY_CURRENT_USER = 0x80000001,
            HKEY_LOCAL_MACHINE = 0x80000002,
            HKEY_USERS = 0x80000003,
            HKEY_CURRENT_CONFIG = 0x80000005
        }

        public enum RegType
        {
            REG_SZ = 1,
            REG_EXPAND_SZ,
            REG_BINARY,
            REG_DWORD,
            REG_MULTI_SZ = 7
        }

        class Program
        {
            static void Main(string[] args)
            {
                const string strComputer = "computername";

                ConnectionOptions options = new ConnectionOptions();
                options.Impersonation = ImpersonationLevel.Impersonate;
                options.EnablePrivileges = true;
                options.Username = "username";
                options.Password = "password";

                ManagementScope myScope = new ManagementScope("\\" + strComputer + "\root\default", options);
                ManagementPath mypath = new ManagementPath("StdRegProv");
                ManagementClass mc = new ManagementClass(myScope, mypath, null);

                object oValue = GetValue(mc, RegHive.HKEY_LOCAL_MACHINE, "SOFTWARE\Microsoft\Windows\CurrentVersion", "ProgramFilesDir");
                Console.WriteLine(oValue.ToString());
            }

            public static object GetValue(ManagementClass mc, RegHive hDefKey, string sSubKeyName, string sValueName)
            {
                RegType rType = GetValueType(mc, hDefKey, sSubKeyName, sValueName);

                ManagementBaseObject inParams = mc.GetMethodParameters("GetStringValue");
                inParams["hDefKey"] = hDefKey;
                inParams["sSubKeyName"] = sSubKeyName;
                inParams["sValueName"] = sValueName;

                object oValue = null;

                switch (rType)
                {
                    case RegType.REG_SZ:
                        ManagementBaseObject outParams = mc.InvokeMethod("GetStringValue", inParams, null);

                        if (Convert.ToUInt32(outParams["ReturnValue"]) == 0)
                        {
                            oValue = outParams["sValue"];
                        }
                        else
                        {
                            // GetStringValue call failed
                        }
                        break;

                    case RegType.REG_EXPAND_SZ:
                        outParams = mc.InvokeMethod("GetExpandedStringValue", inParams, null);

                        if (Convert.ToUInt32(outParams["ReturnValue"]) == 0)
                        {
                            oValue = outParams["sValue"];
                        }
                        else
                        {
                            // GetExpandedStringValue call failed
                        }
                        break;

                    case RegType.REG_MULTI_SZ:
                        outParams = mc.InvokeMethod("GetMultiStringValue", inParams, null);

                        if (Convert.ToUInt32(outParams["ReturnValue"]) == 0)
                        {
                            oValue = outParams["sValue"];
                        }
                        else
                        {
                            // GetMultiStringValue call failed
                        }
                        break;

                    case RegType.REG_DWORD:
                        outParams = mc.InvokeMethod("GetDWORDValue", inParams, null);

                        if (Convert.ToUInt32(outParams["ReturnValue"]) == 0)
                        {
                            oValue = outParams["uValue"];
                        }
                        else
                        {
                            // GetDWORDValue call failed
                        }
                        break;

                    case RegType.REG_BINARY:
                        outParams = mc.InvokeMethod("GetBinaryValue", inParams, null);

                        if (Convert.ToUInt32(outParams["ReturnValue"]) == 0)
                        {
                            oValue = outParams["uValue"] as byte[];
                        }
                        else
                        {
                            // GetBinaryValue call failed
                        }
                        break;
                }

                return oValue;
            }

            public static RegType GetValueType(ManagementClass mc, RegHive hDefKey, string sSubKeyName, string sValueName)
            {
                ManagementBaseObject inParams = mc.GetMethodParameters("EnumValues");
                inParams["hDefKey"] = hDefKey;
                inParams["sSubKeyName"] = sSubKeyName;

                ManagementBaseObject outParams = mc.InvokeMethod("EnumValues", inParams, null);

                if (Convert.ToUInt32(outParams["ReturnValue"]) == 0)
                {
                    string[] sNames = outParams["sNames"] as String[];
                    int[] iTypes = outParams["Types"] as int[];

                    for (int i = 0; i < sNames.Length; i++)
                    {
                        if (sNames[i] == sValueName)
                        {
                            return (RegType)iTypes[i];
                        }
                    }
                    // value not found
                }
                else
                {
                    // EnumValues call failed
                }
            }
        }
    }