C# Extension classes.cs

using System;
using System.Runtime.CompilerServices;

// our code
MyType obj = new();
MyExtension ext = (MyExtension)obj;
ext.MyField = 42;

// other code
ext = (MyExtension)obj;
Console.WriteLine(ext.MyField);

public class MyType;

public class MyExtension
{
    private MyType inner;
    
    private MyExtension(MyType @this)
    {
        inner = @this;
    }
    
    private static ConditionalWeakTable<MyType, MyExtension> _Cache = new();
    
    public static explicit operator MyExtension(MyType @this)
    {
        if(@this is null)
            throw new ArgumentNullException(nameof(@this));
        
        MyExtension extension;
        
        lock(_Cache)
        {
            if(!_Cache.TryGetValue(@this, out extension!))
                _Cache.Add(@this, extension = new(@this));
        }
        
        return extension;
    }
    
    public static explicit operator MyType(MyExtension extension) => extension.inner;
    
    public int MyField;
}