C# Nullable Bool

Are nullable bools efficient? You can use nullable bool types in the C# language, which enable you to represent a null, true and false value in a single variable. However, you want to know if this is an efficient representation of a three-value variable.

This C# article demonstrates a nullable bool. It shows a tri-state enum.

Nullable bool example

Let's get started with the nullable 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. Also, the program shows that each bool? occupies 2 bytes in memory: it has an extra byte of overhead beyond a regular bool.

Program that uses nullable bool [C#]

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);
    }
}

Output

False
2 bytes per bool?

Tristate enum example

Next, let's consider a tristate enum, which can be implemented with a byte backing store. Notice how the semicolon syntax [": byte"] is used after the enum type declaration. The Tristate enum can be set to Tristate.Null, Tristate.True, and Tristate.False. Unlike the nullable bool, though, all three values can be represented in one byte of storage, as the program demonstrates.

Program that uses Tristate byte enum [C#]

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);
    }
}

Output

False
1 byte(s) per Tristate

Summary

Nullable bools can represent three values: null, false and true. However, as we saw here, a custom enum type that uses a byte backing store can more efficiently represent these three values. In many aspects, such as type safety, the approaches are equally effective, but the enum type avoids the overhead associated with wrapping a value type in a generic struct.