Nullable reference types is a powerful feature introduced in C# 8.0 that enhances code safety and reduces the likelihood of null reference exceptions. It allows developers to annotate their reference types to explicitly indicate whether they can be assigned null values or not.

By default, reference types in C# are nullable, meaning they can hold null values. This flexibility, while convenient, can lead to runtime null reference exceptions that can be difficult to debug. Nullable reference types address this issue by introducing compile-time checks to detect possible null references, helping catch potential issues early in the development process.

✅To enable nullable reference types in your C# project, you need to set the nullable context option either in your project file or using compiler flags. Once enabled, you can start annotating your reference types using nullable annotations.

The nullable annotations consist of three states:

*1. Non-nullable types: *
Denoted by a ! suffix (e.g., string!). These types cannot be assigned null values. If a null value is assigned to a non-nullable type, a compiler warning will be generated.

*2. Nullable types: *
Denoted by a ? suffix (e.g., string?). These types can be assigned null values. When working with nullable types, the developer is required to handle potential null values explicitly, either through null checks or using the new null-forgiving operator ! to assert that a nullable reference is not null.

*3. Oblivious types: *
Types without explicit annotations retain the previous behavior. Their nullability is determined based on the context or compiler settings.

Here's an example to illustrate nullable reference types:

#nullable enable

string? nullableString = null;
string nonNullableString = "Hello";

// nullableString can be assigned null
nullableString = "World";

// nonNullableString cannot be assigned null
// Uncommenting the line below would result in a compiler warning
// nonNullableString = null;

// Null check to avoid null reference exception
if (nullableString != null)
{
    int stringLength = nullableString.Length; // No warning, nullableString is checked for null
}

int nonNullableStringLength = nonNullableString.Length; // No warning, nonNullableString is non-nullable

🚀The benefits of using nullable reference types are numerous. They improve code clarity by explicitly expressing the intent of whether a type can be null or not. This helps both the original developers and other team members understand the expected behavior of variables and parameters. Additionally, nullable reference types enable better tooling support, such as code analyzers and editors, to detect potential nullability issues at compile-time.

When adopting nullable reference types in existing codebases, it's common to encounter a mix of nullable and non-nullable types. To help with a smooth transition, C# offers the concept of nullability warnings, which can be configured to generate warnings or errors for specific code elements. This allows developers to gradually annotate existing code and address nullability issues incrementally, ensuring a smooth migration to nullable reference types.