NullReferenceException explained C# with Example



NullReferenceException explained C# with Example

A NullReferenceException is thrown when you try to access a non-static member (property, method, field or 
event) of a reference object but it is null. 
Car myFirstCar = new Car(); 
Car mySecondCar = null; 
Color myFirstColor = myFirstCar.Color; // No problem as myFirstCar exists / is not null 
Color mySecondColor = mySecondCar.Color; //  Throws  a  NullReferenceException 
// as mySecondCar is null and yet we try to access its color. 
To debug such an exception, it's quite easy: on the line where the exception is thrown, you just have to look before 
every '.' or '[', or on rare occasions '('. 
myGarage.CarCollection[currentIndex.Value].Color = theCarInTheStreet.Color; 
Where does my exception come from? Either: 
myGarage is null 
myGarage.CarCollection is null 
currentIndex is null 
myGarage.CarCollection[currentIndex.Value] is null 
theCarInTheStreet is null 
In debug mode, you only have to put your mouse cursor on every of these elements and you will find your null 
reference. Then, what you have to do is understand why it doesn't have a value. The correction totally depends on 
the goal of your method. 
Have you forgotten to instantiate/initialize it? 
myGarage.CarCollection = new Car[10]; 
Are you supposed to do something different if the object is null? 
if (myGarage == null) 
{ 
Console.WriteLine("Maybe you should buy a garage first!"); 
} 
Or maybe someone gave you a null argument, and was not supposed to: 
if (theCarInTheStreet == null) 
{ 
throw new ArgumentNullException("theCarInTheStreet"); 
} 
In any case, remember that a method should never throw a NullReferenceException. If it does, that means you 
have forgotten to check something. 
 

when converting string to other types 

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