null_conditional_operator.cs

class Engine
{
    public bool IsRunning { get; private set; } 
    public void Start() { }        
    public void Stop() { }
}

class Machine
{
    public Engine Engine { get; }
}

public class Program
{
    static void Main()
    {
        Machine notInitializedMachine = null;
        
        // the problem
        try {
            notInitializedMachine.Engine.Start();
        } catch( NullReferenceException ) {
            Console.WriteLine("this ends in an NullReferenceException....");
        }
        
        
        // nothing happens - the machine instance is initialized, 
        // but the engine property is null --> no exception!
        notInitializedMachine = new Machine();
        notInitializedMachine?.Engine?.Start();   
        
        
        /*
            let's assume we just want to know if the engine is running. 
            In case the machine doesn't have an engine (yet), it should 
            simply be false.
        
            this will not work, because IsRunning is bool? in this context:
            bool IsEngineRunning = notInitializedMachine?.Engine?.IsRunning;
            
            -> okay - null coalescing operator to the rescue
        */
        
        bool IsEngineRunning = notInitializedMachine?.Engine?.IsRunning ?? false;
        Console.WriteLine($"{nameof(IsEngineRunning)}: {IsEngineRunning}"); 
        // --> IsEngineRunning: False
        
        
        /*
            without the null conditional & coalescing operators it may look like this:
        */
        IsEngineRunning = false;
        if(notInitializedMachine != null)
        {
            if(notInitializedMachine.Engine != null)
            {
                IsEngineRunning = notInitializedMachine.Engine.IsRunning;
            }
        }
        Console.WriteLine($"{nameof(IsEngineRunning)}: {IsEngineRunning}"); 
        // --> IsEngineRunning: False
    }
}