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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

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