Fallback in my implementation instead of using decorator

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C#

decorator

Currently I am having an sample application that use to render a photo with and without watermark. I have two versions of the application: One with Decorator applied and the other one is without Decorator (kindly see the code below).

I am struggling at what is the disadvantages in my own implementation. Any explanation would be appreciated.

public interface IRender
{
    void Render();
}
 
public class Photo
{
    IRender iRender;
    public void SetRender(IRender iRenderParam)
    {
        iRender = iRenderParam;
    }
 
    public void RenderPhoto()
    {
        iRender.Render();
    }
}
 
public class NormalRender : IRender
{
    public void Render()
    {
        Console.WriteLine("Normal Render");
    }
}
 
public class WaterMarkRender : IRender
{
    private void RenderWaterMark()
    {
        Console.WriteLine("Render WaterMark");
    }
    public void Render()
    {
        RenderWaterMark();
    }
}
 
public class Program
{
    static void Main(string[] args)
    {
        Photo photo = new Photo();
        photo.SetRender(new NormalRender());
        photo.RenderPhoto();
 
        photo.SetRender(new WaterMarkRender());
        photo.RenderPhoto();
 
        Console.ReadLine();
    }
}

What I have tried:

I have inspected my own implementation and cannot figure it own the differences