Null Object Pattern

Null Object Pattern 

Gamma Categorization: Behavioral Design Pattern
Summary:  A no-op object that conforms to the required interface, satisfying a dependency requirement of some other object.

Sample Code

Problem: We want to be able to apply a null filter.

Code

public class Photo

{

    public int Exposure { get; set; } = 0;

    public int Saturation { get; set; } = 0;

    public int Contrast { get; set; } = 0;

    public override string ToString()

    {

        return $"Exposure: {Exposure}, Contrast: {Contrast}, Saturation: {Saturation}";

    }

}

public class PhotoEditor

{

    public void ApplyFilter(Photo photo, IFilter filter) => filter.ApplyFilter(photo);

}

public interface IFilter

{

    void ApplyFilter(Photo photo);

}

public class ImpactFilter : IFilter

{

    public void ApplyFilter(Photo photo) 

    { 

        photo.Saturation += 1;

        photo.Contrast += 2;

    }

}

public class NullFilter : IFilter

{

    public void ApplyFilter(Photo photo) { }

}

Usage

var photo = new Photo();

System.Console.WriteLine(photo);

var photoEditor = new PhotoEditor();

photoEditor.ApplyFilter(photo, new ImpactFilter());

System.Console.WriteLine(photo);

photoEditor.ApplyFilter(photo, new NullFilter());

System.Console.WriteLine(photo);

Output

Exposure: 0, Contrast: 0, Saturation: 0

Exposure: 0, Contrast: 2, Saturation: 1

Exposure: 0, Contrast: 2, Saturation: 1