Nullable

With nullable types in C# programs, every value type, like int, can be null. We wrap an int inside a nullable struct to have a null int.

Nullable types are constructed by specifying the question mark after a value type in a declarator statement. The nullable int is specified with the syntax "int?"

Int example

A nullable variable is specified with a "?" character. We can use a question mark (at the end of a value type) to transform the type into a nullable type.

using System;

//
// Create a local variable of type nullable integer.
// ... It is initially assigned to null.
// ... The HasValue property is false.
//
int? value = null;
Console.WriteLine(value.HasValue);
//
// Assign the nullable integer to a constant integer.
// ... The HasValue property is now true.
// ... You can access the Value property as well.
//
value = 1;
Console.WriteLine(value.HasValue);
Console.WriteLine(value.Value);
Console.WriteLine(value);
if (value == 1)
{
    Console.WriteLine("True");
}
False
True
1
1
True

Bool example

To use a nullable bool, use the type "bool?" with the trailing question mark. This is a struct that contains a bool. The "bool?" can be set to null, true and false.

using System;

class Program
{
    static void Main()
    {
        bool? tristate = null;
        tristate = true;
        tristate = false;

        Console.WriteLine(tristate);

        long m1 = GC.GetTotalMemory(false);
        bool?[] b1 = new bool?[100000];
        long m2 = GC.GetTotalMemory(false);
        b1[0] = false;

        Console.WriteLine("{0} bytes per bool?", (m2 - m1) / 100000);
    }
}
False
2 bytes per bool?

Tri-state enum

We consider a tri-state enum, which can be implemented with a byte backing store. Notice how the semicolon syntax ": byte" is used after the enum type declaration.

using System;

class Program
{
    enum Tristate : byte
    {
        Null = 0,
        True = 1,
        False = 2
    }

    static void Main()
    {
        Tristate tristate = Tristate.Null;
        tristate = Tristate.True;
        tristate = Tristate.False;

        Console.WriteLine(tristate);

        long m1 = GC.GetTotalMemory(false);
        Tristate[] b1 = new Tristate[100000];
        long m2 = GC.GetTotalMemory(false);
        b1[0] = Tristate.False;

        Console.WriteLine("{0} byte(s) per Tristate", (m2 - m1) / 100000);
    }
}
False
1 byte(s) per Tristate

DateTime example

DateTime is a struct, so it cannot directly be null, but we can have a nullable DateTime wrapper. We use the question mark syntax, which results in the type "DateTime?"

using System;

class Program
{
    static void Main()
    {
        //
        // Declare a nullable DateTime instance and assign to null.
        // ... Change the DateTime and use the Test method.
        //
        DateTime? value = null;
        Test(value);
        value = DateTime.Now;
        Test(value);
        value = DateTime.Now.AddDays(1);
        Test(value);
        //
        // You can use the GetValueOrDefault method on nulls.
        //
        value = null;
        Console.WriteLine(value.GetValueOrDefault());
    }

    static void Test(DateTime? value)
    {
        //
        // This method uses the HasValue property.
        // ... If there is no value, the number zero is written.
        //
        if (value.HasValue)
        {
            Console.WriteLine(value.Value);
        }
        else
        {
            Console.WriteLine(0);
        }
    }
}
0
9/29/2009 9:56:21 AM
9/30/2009 9:56:21 AM
1/1/0001 12:00:00 AM

Nullable struct

We can directly access the Nullable generic struct. Here we use a Nullable int by specifying the Nullable type. This syntax has the same effect, but is more verbose.

using System;

class Program
{
    static void Main()
    {
        // Use Nullable directly.
        Nullable<int> test = 100;

        if (test.HasValue)
        {
            Console.WriteLine("HAS VALUE: {0}", test.Value);
        }

        // Set Nullable int to null.
        test = null;

        if (test.HasValue)
        {
            Console.WriteLine("NOT REACHED");
        }
    }
}
HAS VALUE: 100

Memory usage

This program calculates the memory usage for allocating a large array of nullable integers. You can declare a nullable type array using int?[] as the type.

using System;

class Program
{
    static void Main()
    {
        //
        // Compute the memory usage for a nullable type integer.
        // ... The program allocates one million nullable int structs.
        //
        const int size = 1000000;
        long b1 = GC.GetTotalMemory(true);
        int?[] array1 = new int?[size];
        long b2 = GC.GetTotalMemory(true);
        array1[0] = null;
        Console.WriteLine((b2 - b1) / (double)size);
    }
}
8.000016

Notes, nullables

With "null," we can indicate an int is invalid, missing or uninitialized. No special values (like -1) are needed. This can make code more reliable.

Implementation

When you use a nullable type, the C# compiler actually uses the Nullable T struct. The T refers to the value type you are using (such as int).

A summary

Nullable types are value types that are wrapped inside the nullable type. They can be useful when you want to add another state (invalid or uninitialized) to a value type.