C# Null object design pattern

Question

I am looking for a small sample code for Null object design pattern in c#.

I found a sample code from here https://www.tutorialspoint.com/design_pattern/null_object_pattern.htm
which looks fine where factory is implemented to do but that code is in java. so how could I have the same code in c#?

looking for few sample code to implement Null object design pattern using different approach. thanks

Answer 1

Steps are the same in C#, it's just about different syntax and language features. So considering the following steps:

1. Create the abstract class or interface or your object
2. Create concrete implementation
3. Create the null implementation by overriding the methods and returning proper values for the null object

You can see an example following above steps to implement Null Object pattern in C#, but before jumping to the example, you may want to learn more about the following C# language features which may help you avoid null exceptions:

• ?? and ??= Null-coalescing and null-coalescing assignment operators
• ?. and ?[] Null-conditional operators
• Nullable reference types

Example C# Null Object Pattern

Here is the interface for animal:

public interface IAnimal  
{  
    string GetName();  
    string MakeSound();  
}  

And two concrete implementations, for duck and cat:

public class Duck : IAnimal  
{  
    public string GetName()  
    {  
        return "Duck";  
    }  
    public string MakeSound()  
    {  
        return "Quack quack!";  
    }  
}  
public class Cat : IAnimal  
{  
    public string GetName()  
    {  
        return "Cat";  
    }  
    public string MakeSound()  
    {  
        return "Meow meow!";  
    }  
}  

Now I'll implement the null object as a singleton:

public sealed class NullAnimal : IAnimal  
{  
    private static readonly IAnimal nullAnimal = new NullAnimal();  
    private NullAnimal() { }  
    public string MakeSound()  
    {  
        return string.Empty;  
    }  
    public string GetName()  
    {  
        return String.Empty;  
    }  
    public static IAnimal Instance  
    {  
        get  
        {  
            return nullAnimal;  
        }  
    }  
}  

Then I'll create a factory to return the animal, based on the name:

public class AnimalFactory  
{  
    public static IAnimal GetAnimal(string name)  
    {  
        if (name == "duck")  
            return new Duck();  
        else if (name == "cat")  
            return new Cat();  
        else  
            return NullAnimal.Instance;  
    }  
}  

And the usage:

var animals = new[] { "duck", "cat", "unknown" };  
foreach (var name in animals)  
{  
    var animal = AnimalFactory.GetAnimal(name);  
    Console.WriteLine($"{animal.GetName()} says: {animal.MakeSound()}");  
}  

Which returns:

Duck says: Quack quack!  
Cat says: Meow meow!  
 says:   

Answer 2

See the following where the null-coalescing operator ?? in the closing example shows a Null Object Pattern.