C# Academy · Lesson

Nullable Value Types

int? and the Nullable struct.

Lesson 1 of 413 steps

Nullable Value Types is a free C# Academy lesson on CoddyKit. This is lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C# Academy learning path, and your progress syncs across the web and the CoddyKit app. The C# Academy course includes 4 lessons in total.

Why Nullable Value Types?

Value types like int, bool, and DateTime always hold a value. An int defaults to 0, never "no value".

Sometimes you need to represent missing data, such as an unanswered survey field or an empty database column. Nullable value types let a value type also hold null.

Declaring with int?

Add a ? after a value type to make it nullable. int? can hold any integer or null.

This is the most common way you will write nullable value types in everyday C# code.

int? age = null;
int count = 5;
age = count;
System.Console.WriteLine(age);

Nullable<T> Behind the Scenes

The ? syntax is shorthand. int? is exactly the same type as Nullable<int>.

Nullable<T> is a struct that wraps a value type and adds a flag tracking whether a value is present.

Nullable<int> a = 10;
int? b = 10;
System.Console.WriteLine(a == b);

HasValue and Value

Every nullable exposes two properties. HasValue is true when a value is present, and Value returns the underlying value.

Reading Value when HasValue is false throws an InvalidOperationException, so check first.

int? score = 42;
if (score.HasValue)
{
    System.Console.WriteLine(score.Value);
}

A Complete Example

Here is a full program. It assigns a value, checks HasValue, and prints the result.

Run it to see how a nullable behaves when it actually holds data.

using System;

class Program
{
    static void Main()
    {
        int? temperature = 21;
        if (temperature.HasValue)
            Console.WriteLine("Temp: " + temperature.Value);
        else
            Console.WriteLine("No reading");
    }
}

When It Is Null

Assigning null makes HasValue return false. The else branch then runs.

This pattern is how you safely handle optional readings without crashing.

using System;

class Program
{
    static void Main()
    {
        int? temperature = null;
        if (temperature.HasValue)
            Console.WriteLine(temperature.Value);
        else
            Console.WriteLine("No reading");
    }
}

GetValueOrDefault

GetValueOrDefault() returns the value if present, otherwise the default for that type (such as 0 for int).

You can also pass your own fallback: GetValueOrDefault(100) returns 100 when null.

int? quantity = null;
int result = quantity.GetValueOrDefault(100);
System.Console.WriteLine(result);

Implicit Conversion to Nullable

A regular int converts to int? automatically, because every plain value is also a valid nullable value.

The reverse is not automatic: going from int? back to int requires an explicit cast or a fallback, since the nullable might be null.

int plain = 7;
int? wrapped = plain;
int back = (int)wrapped;
System.Console.WriteLine(back);

Nullable with Other Value Types

Any value type works: bool?, double?, DateTime?, and even custom structs.

A bool? has three states: true, false, and null, which is useful for unanswered yes or no questions.

bool? agreed = null;
double? price = 9.99;
System.Console.WriteLine(agreed.HasValue);
System.Console.WriteLine(price.Value);

Reference Types Are Already Nullable

Classes like string and arrays can already be null, so they do not need the Nullable<T> wrapper.

The ? on reference types (like string?) means something different: it is a nullable-reference annotation, not the Nullable<T> struct.

Comparing Nullables

Comparisons work, but remember null behaves specially. Two nulls are equal, and any ordered comparison involving null returns false.

So null < 5 is false, and null > 5 is also false.

int? x = null;
System.Console.WriteLine(x < 5);
System.Console.WriteLine(x == null);

Quick Check

Test your understanding of nullable value types.

Answer it in the interactive lesson

Recap

Nullable value types let value types also hold null. Write them as int?, which equals Nullable<int>.

Use HasValue to test, Value to read (only when present), and GetValueOrDefault() for a safe fallback. Reference types are already nullable on their own.

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Frequently Asked Questions

Is the “Nullable Value Types” lesson free?

Yes — the full text of “Nullable Value Types” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C# Academy course, upgrade to CoddyKit PRO. The C# Academy course includes 4 lessons in total.

What will I learn in “Nullable Value Types”?

int? and the Nullable struct. You practise C# Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start C# Academy?

No prior experience is required. C# Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 1 of 4.

How long does the “Nullable Value Types” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this C# Academy lesson?

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