First example. It is useful to look at an example of a short program that causes this exception to be raised. The program explicitly assigns the string reference variable to the null literal.
Note This means that the string variable does not point to any object on the managed heap. It is equivalent to a null pointer.
Main The string reference variable is assigned to the null literal. Next, the Length property is accessed on the string reference.
Info The program compiles correctly, but always throws an execution. You cannot access an instance property like Length on a null reference.
C# program that raises NullReferenceException
static void Main()
string value = null;
if (value.Length == 0) // <-- Causes exception
Console.WriteLine(value); // <-- Never reached
System.NullReferenceException: Object reference not set to an instance of an object.
at Program.Main() in C:\Users\...
Example 2. We see one way you can prevent the null reference exception from being thrown. Often in programs, methods receive reference value parameters. The parameters contain the storage location of an object on the managed heap.
However If the parameter points to null, the compiler will not know this at compile-time. You must check for null at the method start.
Info An array containing two elements is passed to the Test method. The Test method checks its parameter against the null literal.
Therefore Test() will not throw when passed a null reference, as we see in the latter part of the Main method.
C# program that checks for null in method
static void Main()
// Create an array and use it in a method.
int array = new int;
array = 1;
array = 2;
// Use null reference in a method.
array = null;
Test(array); // <-- Won't crash
static void Test(int array)
if (array == null)
// You can throw an exception here, or deal with the argument.
int rank = array.Rank;
Discussion. Exception handling is a complex topic. And developers have different strategies. However, the common theme to the best approaches is to always log the exceptions and work to prevent them from happening in normal runtime operation.
Also In many programs you can use ArgumentNullException to notify the caller of errors.
Handling null references. If you have total control over the callers of a method and it can never be called with a null parameter, it is preferable not to check the parameter for null.
But For libraries and code APIs that will be used by others, more careful parameter checking to avoid NullReferenceExceptions is best.
Summary. We looked the NullReferenceException error reporting class in the System namespace, using the C# language. You will encounter this exception when you attempt to access a member on a null variable.