C# NullReferenceException

A NullReferenceException is one of the most common runtime exceptions in C#. It occurs when you try to access a member (property, method, or field) of an object reference that is null.

When NullReferenceException Occurs

This exception is thrown when −

• You call a method on a null reference

• You access or modify a property of a null reference

• You get the length of a null array or string

• You index into a null array

Example: Accessing Null String

using System;

class Demo {
   static void Main() {
      string str = null;

      if (str.Length > 0) {
         Console.WriteLine(str);
      }
   }
}

The output of the above code is −

Unhandled Exception:
System.NullReferenceException: Object reference not set to an instance of an object
   at Demo.Main()

Example: Accessing Null Array

using System;

class Demo {
   static void Main() {
      int[] numbers = null;
      
      Console.WriteLine("Array length: " + numbers.Length);
   }
}

The output of the above code is −

Unhandled Exception:
System.NullReferenceException: Object reference not set to an instance of an object
   at Demo.Main()

How to Prevent NullReferenceException

Using Null Checks

using System;

class Demo {
   static void Main() {
      string str = null;

      if (str != null && str.Length > 0) {
         Console.WriteLine(str);
      } else {
         Console.WriteLine("String is null or empty");
      }

      int[] numbers = null;
      if (numbers != null) {
         Console.WriteLine("Array length: " + numbers.Length);
      } else {
         Console.WriteLine("Array is null");
      }
   }
}

The output of the above code is −

String is null or empty
Array is null

Using Null-Conditional Operator (?.) - C# 6.0+

using System;

class Person {
   public string Name { get; set; }
}

class Demo {
   static void Main() {
      Person person = null;
      
      // Safe access using null-conditional operator
      string name = person?.Name ?? "Unknown";
      Console.WriteLine("Name: " + name);
      
      // Safe method call
      int? length = person?.Name?.Length;
      Console.WriteLine("Name length: " + (length?.ToString() ?? "N/A"));
   }
}

The output of the above code is −

Name: Unknown
Name length: N/A

Common Scenarios and Prevention

Conclusion

NullReferenceException occurs when attempting to access members of a null reference. Prevent it by checking for null before accessing object members, using the null-conditional operator (?.), or ensuring proper object initialization. Always validate object references before use to avoid runtime exceptions.