Use the NullReferenceException. : Predefined Exception « Language Basics « C# / CSharp Tutorial

1. C# / CSharp Tutorial
2. Language Basics
3. Predefined Exception

using System;  
  
class MainClass {   
  public static void Main() {   

    try { 
      string str = null;
      Console.WriteLine(str.ToString());
      
    } catch (NullReferenceException) { 
      Console.WriteLine("NullReferenceException!"); 
      Console.WriteLine("fixing...\n"); 
 
    } 
 
  }  
}

NullReferenceException!
fixing...