CSharp Nullability Basics

C# Nullability: Basic Null Checks and Operators

This lesson introduces nullable value and reference types, and how to handle null safely.

Related wiki pages:

• Exceptions and robustness: CSharp-Exceptions-TryCatch.md
• Properties and initialization: Lessons.md (to be renamed to a properties-focused file)

Official docs:

• Nullable reference types: https://learn.microsoft.com/dotnet/csharp/nullable-references

---

1. Nullable value types

int? maybeValue = null; // nullable int

if (maybeValue.HasValue)
{
    Console.WriteLine(maybeValue.Value);
}
else
{
    Console.WriteLine("No value");
}

int? is shorthand for Nullable<int>.

---

2. Basic null check

public static void PrintIfNotNull(string? input)
{
    if (input != null)
    {
        Console.WriteLine($"Input: {input}");
    }
    else
    {
        Console.WriteLine("Input is null.");
    }
}

---

3. Null-conditional and null-coalescing operators

string? text = null;

int? length = text?.Length;           // null if text is null
int lengthOrZero = text?.Length ?? 0; // 0 if text is null

string result = text ?? "default";   // "default" if text is null

• ?. stops evaluation if the left side is null.
• ?? provides a fallback value if the left side is null.

---

4. Good practices

• Avoid dereferencing possibly-null values without checks.
• Enable nullable annotations in your project (<Nullable>enable</Nullable> in the project file) for better compiler warnings.
• Combine null checks with exception handling where appropriate (CSharp-Exceptions-TryCatch.md).

---

5. Next steps

• Apply these operators in your own classes and property accessors.
• Explore advanced patterns like ArgumentNullException.ThrowIfNull in .NET 6+.