Digging Into Nullable Reference Types in C#


Nullable reference types were introduced in C# 8.0 to enhance code safety by catching potential null reference exceptions at compile-time. This feature empowers developers to write more robust and reliable code when dealing with reference types. In this article, we will delve into the concept of nullable reference types and provide detailed examples to showcase their usage.

1. Introduction to Nullable Reference Types

In C#, reference types, such as classes and strings, can be assigned the value null to indicate the absence of an object reference. Nullable reference types introduce the ability to declare whether a reference type variable can be null or non-null, allowing the compiler to catch null reference exceptions at compile-time.

2. Nullable Value Types vs. Nullable Reference Types

Before we dive into examples, it’s important to clarify the distinction between nullable value types and nullable reference types.

  • Nullable Value Types: These enable value types (int, float, bool, etc.) to accommodate a null-like value by using the Nullable<T> struct or appending a ? to the type name (e.g., int?, float?).

  • Nullable Reference Types: Applicable to reference types, they indicate whether a reference can hold a null value or must be non-null. This is accomplished by appending a ? to the type name (e.g., string?, Person?).

3. Using Nullable Reference Types

Let’s start with a simple example that demonstrates the use of nullable reference types:

#nullable enable

using System;

class Program
{
    static void Main(string[] args)
    {
        string? name = "Alice"; // Nullable reference type

        int nameLength = name.Length; // Warning: Possible null reference
        Console.WriteLine($"Name length: {nameLength}");
    }
}

In this snippet, string? name is a nullable reference type. Attempting to access the Length property without null checking triggers a warning since name can be null.

4. Nullable Reference Types in Classes

You can apply nullable reference types to class properties:

#nullable enable

class Person
{
    public string FirstName { get; set; }
    public string? LastName { get; set; } // Nullable reference type
}

In this Person class, FirstName is non-nullable by default, while LastName is nullable. This expresses that LastName can be either a string or null.


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5. Nullable Reference Types with Methods

Methods can accept and return nullable reference types:

#nullable enable

class Program
{
    static void Main(string[] args)
    {
        string? name = GetFullName("Alice", "Smith");

        Console.WriteLine($"Full Name: {name}");
    }

    static string? GetFullName(string firstName, string? lastName)
    {
        return lastName != null ? $"{firstName} {lastName}" : null;
    }
}

Here, the GetFullName method accepts a nullable reference type lastName and can return either a full name or null.

6. Handling Nullable Reference Types

Proper handling of nullable reference types is essential to avoid null reference exceptions:

#nullable enable

class Program
{
    static void Main(string[] args)
    {
        Person person1 = new Person { FirstName = "Alice", LastName = "Smith" };
        Person person2 = new Person { FirstName = "Bob" };

        PrintPersonDetails(person1);
        PrintPersonDetails(person2);
    }

    static void PrintPersonDetails(Person person)
    {
        string lastName = person.LastName ?? "Unknown";
        Console.WriteLine($"Name: {person.FirstName} {lastName}");
    }
}

In this code, the PrintPersonDetails method safely handles the nullable LastName property using the null coalescing operator (??).

7. Null-Forgiving Operator (!)

The null-forgiving operator (!) allows you to assert that a nullable reference type won’t be null at a certain point:

#nullable enable

class Program
{
    static void Main(string[] args)
    {
        string? name = "Alice";

        int nameLength = name!.Length; // No warning, asserting not null
        Console.WriteLine($"Name length: {nameLength}");
    }
}

Here, name! asserts that name is not null, suppressing any warnings.


By embracing nullable reference types, you can significantly enhance the safety and reliability of your C# code. This feature encourages proactive null safety and helps prevent null reference exceptions, resulting in more robust and maintainable applications.

We hope this article has provided you with a comprehensive understanding of nullable reference types and how to use them effectively in various scenarios.

Remember to enable nullable reference types at the project level by using the #nullable enable directive in your source files or project settings.

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