NullreferenceException in C#

The NullReferenceException is an exception that will be thrown while accessing a null object.

The following example shows the code that throws the NullReferenceException:

public class Program
{
	public static void Main()
	{
		IList<string> cities = null;
		DisplayCities(cities);
	}

	public static void DisplayCities(IList<string> cities)
	{
		foreach (var city in cities)
		{
			Console.WriteLine(city);
		}
	}
}

In the above example, a NullReferenceException will be thrown in the DisplayCities() function while accessing cities list using a foreach loop because the cities list is null. If the caller of the DisplayCities() function pass a null IList value then it will raise a NullReferenceException.

Solutions to fix the NullReferenceException

To prevent the NullReferenceException exception, check whether the reference type parameters are null or not before accessing them.

Solution 1: Check whether an object contains a null value or not using an if condition, as shown below:

public class Program
{
	public static void Main()
	{
		IList<string> cities = null;
		DisplayCities(cities);
	}

	public static void DisplayCities(IList<string> cities)
	{
		if (cities == null) //check null before accessing
		{
			Console.WriteLine("No Cities");
			return;
		}

		foreach (var city in cities)
		{
			Console.WriteLine(city);
		}
	}
}

In the above example, if(cities == null) checks whether the cities object is null or not. If it is null then display the appropriate message and return from the function.

Solution 2: In .NET 5, use the null conditional operator ?., as shown below:

public class Program
{
	public static void Main()
	{
		Student std = null;
		Display(std);
		
		std = new Student(){ FirstName = "Vinod", LastName = "Dhar"};
		Display(std);
	}

	public static void Display(Student std)
	{
		Console.WriteLine("Student Info:");
		Console.WriteLine(std?.FirstName); //use ?. operator 
		Console.WriteLine(std?.LastName); // use ?. operator
	}
}

public class Student
{
	public int Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

In the above example, std?.FirstName is like if(std != null) std.FirstName. The std?.FirstName checks if std is not null then only access the FirstName property.

Solution 3: In .NET 4.x and above versions, use Null-Coalescing operator ?? to prevent an exception, as shown below:

public class Program
{
	public static void Main()
	{
		IList<string> cities = null;
		DisplayCities(cities);
	}

	public static void DisplayCities(IList<string> cities)
	{

		foreach (var city in cities?? new List<string>())
		{
			Console.WriteLine(city);
		}
	}
}

In the above example, ?? is a null coalescing operator that checks if an object is null or not, if it is null then create an object. The cities ?? new List<string>() creates a new list object if a cities is null in foreach loop. Thus, the NullReferenceException will be not be raised.