NullObject

Null Object

The intent of a Null Object is to encapsulate the absense of an object by providing a substitutable alternative that offers suitable default do nothing behavior.

Problem

• An object reference may be optionally null and
• This reference must be checked before every use and
• The result of a null check is to do nothing or assign a suitable default value.

if (log != null) {
        log.Write(request);
}

// more statements

if (log != null) {
        log.Write(request);
}

Solution

• Provide a class derived from the object reference's type and
• Implement all its methods to do nothing or provide default results and
• Use an instance of this class whenever the object reference would have been null

Definition

    interface ILog
    {
        void Write(string message);
    }

    class NullLog : ILog
    {
        public void Write(string message)
        {
            // do nothing
        }
    }

Usage

ILog log = new NullLog();

// somewhere later
log.Write(request);

C# Null Propagation

log?.Write(request);

Note

Null-Object cannot be used for Immutable classes (eg. String).