C# Academy · Lesson

Introducing Nullable Value Types

Declare int?, double? and understand Nullable .

Introducing Nullable Value Types is a free C# Academy lesson on CoddyKit — 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, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Value Types Cannot Be Null

Normally an int always holds a number; it can never be null. Sometimes you need to represent "no value", such as a missing measurement.

using System;

class Program {
    static void Main() {
        int count = 0;
        Console.WriteLine(count);
    }
}

Nullable with ?

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

using System;

class Program {
    static void Main() {
        int? maybe = null;
        Console.WriteLine(maybe.HasValue);
        maybe = 42;
        Console.WriteLine(maybe);
    }
}

Other Nullable Types

Any value type can be nullable: double?, bool?, DateTime?, and so on.

using System;

class Program {
    static void Main() {
        double? temp = null;
        bool? flag = true;
        Console.WriteLine(temp.HasValue + " " + flag);
    }
}

Nullable<T> Spelled Out

int? is just shorthand for the struct Nullable<int>. Both compile to exactly the same type.

using System;

class Program {
    static void Main() {
        Nullable<int> a = 10;
        int? b = 10;
        Console.WriteLine(a == b);
    }
}

Assigning Null

You can freely assign null to a nullable variable and reassign a real value later.

using System;

class Program {
    static void Main() {
        int? id = 100;
        Console.WriteLine(id);
        id = null;
        Console.WriteLine(id.HasValue);
    }
}

Printing a Null Nullable

Printing a nullable that is null produces an empty string, not the word null.

using System;

class Program {
    static void Main() {
        int? value = null;
        Console.WriteLine("[" + value + "]");
    }
}

Implicit Conversion to Nullable

A plain int converts to int? automatically because every int is a valid nullable int.

using System;

class Program {
    static void Main() {
        int x = 7;
        int? y = x;
        Console.WriteLine(y);
    }
}

Nullable in Method Returns

A method can return a nullable type to signal "found a value" versus "found nothing".

using System;

class Program {
    static int? FindEven(int[] nums) {
        foreach (int n in nums)
            if (n % 2 == 0) return n;
        return null;
    }
    static void Main() {
        Console.WriteLine(FindEven(new[] {1, 3, 4}));
    }
}

Comparing Nullables

Equality works as expected: two nullables are equal if both are null or both hold the same value.

using System;

class Program {
    static void Main() {
        int? a = null;
        int? b = null;
        Console.WriteLine(a == b);
        int? c = 5;
        Console.WriteLine(c == 5);
    }
}

Why Use Nullable Types

Nullable value types model optional data cleanly, such as a database column that allows NULL or a field a user left blank.

using System;

class Program {
    static void Main() {
        int? userAge = null;
        string msg = userAge.HasValue ? "Age set" : "Age unknown";
        Console.WriteLine(msg);
    }
}

Nullable Is a Struct

Nullable<T> itself is a value type, so a nullable variable is never a reference even though it can be null.

using System;

class Program {
    static void Main() {
        int? n = 3;
        Console.WriteLine(n.GetType().Name);
    }
}

Quick Check

Test your understanding of nullable value types.

Recap

Value types like int cannot be null by default. Adding ? creates a nullable type (int?), which is shorthand for Nullable<T>.

Nullable types let you represent missing or optional data, convert implicitly from the underlying type, and are themselves structs.

Frequently asked questions

Is the “Introducing Nullable Value Types” lesson free?

Yes — the full text of “Introducing Nullable Value Types” is free to read here on the web, and the C# Academy course includes 4 lessons in total. 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.

What will I learn in “Introducing Nullable Value Types”?

Declare int?, double? and understand Nullable . 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; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Introducing 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?

Yes. Every C# Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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