How to capture null reference exception in C#?

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

What Causes NullReferenceException?

The exception is thrown when you try to access members of an object that has not been initialized or has been set to null −

string str = null;
int length = str.Length; // Throws NullReferenceException

Using Null Checks

The most straightforward way to prevent NullReferenceException is to check for null before accessing object members −

using System;

class Program {
   static void Main() {
      int[] arr = new int[5] {1, 2, 3, 4, 5};
      DisplayArrayInfo(arr);

      arr = null;
      DisplayArrayInfo(arr);
   }

   static void DisplayArrayInfo(int[] arr) {
      if (arr == null) {
         Console.WriteLine("Array is null");
         return;
      }
      Console.WriteLine("Array rank: " + arr.Rank);
      Console.WriteLine("Array length: " + arr.Length);
   }
}

The output of the above code is −

Array rank: 1
Array length: 5
Array is null

Using Try-Catch Block

You can also use exception handling to catch and handle NullReferenceException −

using System;

class Program {
   static void Main() {
      string str = null;
      
      try {
         int length = str.Length;
         Console.WriteLine("Length: " + length);
      }
      catch (NullReferenceException ex) {
         Console.WriteLine("Caught NullReferenceException: " + ex.Message);
      }
      
      Console.WriteLine("Program continues executing");
   }
}

The output of the above code is −

Caught NullReferenceException: Object reference not set to an instance of an object.
Program continues executing

Using Null-Conditional Operator

C# provides the null-conditional operator (?.) which safely accesses members only if the object is not null −

using System;

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

class Program {
   static void Main() {
      Person person = null;
      
      // Safe access using null-conditional operator
      string name = person?.Name;
      int? age = person?.Age;
      
      Console.WriteLine("Name: " + (name ?? "Not available"));
      Console.WriteLine("Age: " + (age?.ToString() ?? "Not available"));
      
      // Method chaining with null-conditional
      int? nameLength = person?.Name?.Length;
      Console.WriteLine("Name length: " + (nameLength?.ToString() ?? "Not available"));
   }
}

The output of the above code is −

Name: Not available
Age: Not available
Name length: Not available

Best Practices

Conclusion

Preventing NullReferenceException is crucial for robust C# applications. Use null checks for explicit control, null-conditional operators for concise code, and try-catch blocks as a safety net. Always validate object references before accessing their members to avoid runtime crashes.