NullReferenceException

This error indicates that you are trying to access member fields, or function types, on an object reference that points to null.

An exception indicates a flaw in the code, or an invalid configuration or missing file. Often a code change is needed to correct the C# program.

First example

This program causes a NullReferenceException to be raised. The program explicitly assigns the string reference variable to the null literal.

using System;

class Program
{
    static void Main()
    {
        string value = null;
        if (value.Length == 0) // <-- Causes exception
        {
            Console.WriteLine(value); // <-- Never reached
        }
    }
}
Unhandled Exception:
System.NullReferenceException: Object reference not set to an instance of an object.
at Program.Main() in C:\Users\...

Example 2

We can prevent a NullReferenceException. If the parameter points to null, the compiler will not know this at compile-time. You must check for null at the method start.

using System;

class Program
{
    static void Main()
    {
        // Create an array and use it in a method.
        int[] array = new int[2];
        array[0] = 1;
        array[1] = 2;
        Test(array);

        // 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.
            return;
        }
        int rank = array.Rank;
        Console.WriteLine(rank);
    }
}

Discussion

Exception handling is a complex topic. A common theme to the best approaches is to always log the exceptions and work to prevent them from happening in normal runtime operation.

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.

We looked the NullReferenceException error reporting class. You will encounter this exception when you attempt to access a member on a null variable.