Installer Custom Actions - Cannot Write Registration Key

Question

In the custom action editor, I added custom actions to set and remove process steps. In the properties window, I marked the CustomActionData property as:

/TARGETDIR = "[TARGETDIR]"

I hope the above transmits installation information to a user action.

The custom action seems to fire, but I get the following error message:

"Error 1001. Unable to write registration key" (or something like that, I translated it from my local language).

What am I doing wrong?

using System;
using System.Collections;
using System.Collections.Generic;
using System.ComponentModel;
using System.Configuration.Install;
using System.Linq;
//using System.Windows.Forms;
using Microsoft.Win32;

namespace CustomActions
{
    [RunInstaller(true)]
    public partial class Installer1 : Installer
    {
        public override void Install(IDictionary stateSaver)
        {
            base.Install(stateSaver);

            const string key_path = "SOFTWARE\\VendorName\\MyAppName";
            const string key_value_name = "InstallationDirectory";

            RegistryKey key = Registry.LocalMachine.OpenSubKey(key_path);

            if (key == null)
            {
                key = Registry.LocalMachine.CreateSubKey(key_path);
            }

            string tgt_dir = Context.Parameters["TARGETDIR"];

            key.SetValue(key_value_name, tgt_dir);

        }

        public override void Uninstall(IDictionary savedState)
        {
            base.Uninstall(savedState);

            const string key_path = "SOFTWARE\\VendorName";
            const string key_name = "MyAppName";

            RegistryKey key = Registry.LocalMachine.OpenSubKey(key_path);

            if (key.OpenSubKey(key_name) != null)
            {
                key.DeleteSubKey(key_name);
            }
        }

        public override void Rollback(IDictionary savedState)
        {
            base.Rollback(savedState);
        }


        public Installer1()
        {
            InitializeComponent();
        }
    }
}

Answer 1

Try changing:
RegistryKey key = Registry.LocalMachine.OpenSubKey(key_path);

To:
RegistryKey key = Registry.LocalMachine.OpenSubKey(key_path, Microsoft.Win32.RegistryKeyPermissionCheck.ReadWriteSubTree);