.NET (Core and Framework)

Control ctrlToActivate;

void timer1_tick(object sender, EventArgs e)
{
   if (ctrlToActive == null) return;
   ctrlToActivate.Select();
   ctrlToActivate = null;
}

void tree_afterSelect(object sender, TreeViewEventArgs e)
{
  /* 
   Here you need to map the selected node to a control you wish to activate.
   This is also a good place to hide or display elements of the UI. 
   For my test, I used a tab control, tagged each node in the tree with the index
   of the associated tab page, and always activate the first textbox on the tab page.
  */

   tabControl1.SelectedIndex = int.Parse(e.Node.Tag.ToString());
   foreach (Control c in tabControl1.SelectedTab.Controls)
      if (c is TextBox) { ctrlToActivate = c; return; }
}

MSDN [^]wrote:

An enumeration is a named constant whose underlying type is any integral type except Char. If no underlying type is explicitly declared, Int32 is used. Programming languages typically provide syntax to declare an enumeration that consists of a set of named constants and their values.

Enum provides methods to compare instances of this class, convert the value of an instance to its string representation, convert the string representation of a number to an instance of this class, and create an instance of a specified enumeration and value.

I are troll

C#
[SerializableAttribute]
[ComVisibleAttribute(true)]
public abstract <big>class</big> Enum : ValueType, IComparable, IFormattable, IConvertible

using System.Runtime.InteropServices;

namespace System
{
    // Summary:
    //     Provides the base class for enumerations.
    [Serializable]
    [ComVisible(true)]
    public abstract <big>class</big> Enum : ValueType, IComparable, IFormattable, IConvertible
    {
        members snipped ...
    }
}

MSDN wrote:

An enumeration is a named constant whose underlying type is any integral type except Char.

I are troll

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    enum en { Zero, One, Two, Three };
    enum fr { Nul, Un, Deux, Trois };

    void foo(object obj)
    {
        if (obj is en) dbug("en:{0} => fr:{1}", obj, (fr)obj);
        else if (obj is fr) dbug("fr:{0} => en:{1}", obj, (en)obj);
        else dbug("?:{0} => en:{1}, fr:{2}", obj, (en)obj, (fr)obj);
    }

    
    void dbug(string s, params object[] args)
    {
        if (args.Length > 0) s = string.Format(s, args);
        Debug.WriteLine(s);
    }

    
    void button1_Click(object sender, EventArgs e)
    {
        Random r = new Random();
        for (int i = 0; i < 20; i++)
        {
            object v = r.Next(4);
            if (r.Next(100) < 33)
                foo((en)v);
            else if (r.Next(100) < 50)
                foo((fr)v);
            else foo(v);

            Thread.Sleep(500);
        }
    }
}

Type.GetType("System.Enum").IsClass
Type.GetType("System.Enum").IsInterface
Type.GetType("System.Enum").IsValueType