data.cs

using com.example.mixin;
namespace com.example.data
{
  public class Mixed1 : IMixin 
  {
    public Dictionary<string, object> Property { get; set; }
  }
  
  public class Mixed1 : IMixin 
  {
    public Dictionary<string, object> Property { get; set; }
  }
}

mixin.cs

namespace com.example.mixin 
{
  public interface IMixin
  {
    public Dictionary<string, object> Property { get; set; }
  }
  
  public static class MixinMethods
  {
    public static object ReturnObject(this IMixin self)
    {
      return self;
    }
    
    public static void Merge(this IMixin self, IMixin other)
    {
      foreach(string k in other.Property.Keys)
      {
        self.Property[k] = other.Property[k];
      }
    }
  }
}

Program.cs

using com.example.mixin;
using com.example.data;

namespace com.example.main
{
  public class Program
  {
    public static void Main(string[] args)
    {
      Mixed1 m1 = new Mixed1();
      Mixed2 m2 = new Mixed2();
      
      m1.Property = new Dictionary<string, object>();
      m2.Property = new Dictionary<string, object>();
      
      
      m1.Merge(m2);
      
      Console.WriteLine(m1.ReturnObject());
      Console.WriteLine(m2.ReturnObject());
    }
  }
}