How to use nullable reference types in C#
Nullable reference types, introduced in C# 8.0, help prevent null reference exceptions by making nullability explicit. Enable them with <Nullable>enable</Nullable> in your project file or #nullable enable in code.
A type followed by ? (like string?) indicates it can be null. Types without ? should never be null, and the compiler warns you about potential null dereferences.
The null-forgiving operator ! tells the compiler you know a value isn't null, suppressing warnings. Use it sparingly when you have information the compiler doesn't.
C# Example Code
#nullable enable
using System;
// Non-nullable reference type (cannot be null)
string name = "Alice";
Console.WriteLine($"Length: {name.Length}"); // Safe
// Nullable reference type (can be null)
string? nullableName = null;
// Console.WriteLine(nullableName.Length); // Warning: possible null reference
// Safe null checking
if (nullableName != null)
{
Console.WriteLine($"Length: {nullableName.Length}");
}
// Null-conditional operator
int? length = nullableName?.Length;
Console.WriteLine($"Length: {length ?? 0}");
// Null-forgiving operator (use carefully!)
string? possiblyNull = GetValue();
string definitelyNotNull = possiblyNull!; // Tells compiler it's not null
Console.WriteLine(definitelyNotNull);
// Method with nullable parameters
PrintMessage(null); // OK, parameter is nullable
PrintMessage("Hello");
// Method with non-nullable parameter
// PrintRequired(null); // Warning: passing null to non-nullable parameter
PrintRequired("Required");
static string? GetValue()
{
return "Some value";
}
static void PrintMessage(string? message)
{
// Safe handling of nullable parameter
Console.WriteLine(message ?? "No message");
}
static void PrintRequired(string message)
{
// message is guaranteed to not be null (by contract)
Console.WriteLine($"Required: {message.ToUpper()}");
}
// Properties with nullable reference types
class Person
{
public string Name { get; set; } = ""; // Non-nullable, must initialize
public string? MiddleName { get; set; } // Nullable, can be null
public string LastName { get; set; } = "";
public string GetFullName()
{
return MiddleName != null
? $"{Name} {MiddleName} {LastName}"
: $"{Name} {LastName}";
}
}