5.1. Booleans

One of the core features of any programming language is the ability to conditionally execute a segment of code. This means that a program will run a segment of code only if a given condition is met.

Example

Consider a banking application that can remind you when a bill is due. The application will notify you that a bill is due soon, but only if the bill has not already been paid.

The condition for the above example is: Send a notification of an upcoming bill only if the statement “the bill is unpaid” is true. In order to state something like this in C#, we need to understand how programming languages represent true and false.

5.1.1. Boolean Values

The C# data type for storing true and false values is boolean, named after the British mathematician George Boole.

Fun Fact

George Boole created Boolean Algebra, which is the basis of all modern computer arithmetic.

There are only two boolean values—true and false. C# is case-sensitive, so True and False are not valid boolean values.

Example

Console.WriteLine(true);
Console.WriteLine(true.GetType());
Console.WriteLine(false.GetType());

Console Output

True
System.Boolean
System.Boolean

The values true and false are not strings. If you use quotes to surround booleans ("true" and "false"), those values become strings.

Example

Console.WriteLine(true.GetType());
Console.WriteLine("true".GetType());

Console Output

System.Boolean
System.String

5.1.2. Boolean Conversion

As with the numeric and string data types, the boolean type also has a conversion function, Convert.ToBoolean(). It works similarly to the Int32.Parse() and Double.Parse() methods, attempting to convert a non-boolean value to a boolean. This is very limited as is, but this could be useful if working with user input, specifically the words “true” and “false”.

Try It!

Explore how Convert.ToBoolean() converts various non-boolean values. Explore differnt data types.

Console.WriteLine(Convert.ToBoolean("true"));
Console.WriteLine(Convert.ToBoolean("TRUE"));
Console.WriteLine(Convert.ToBoolean(0));
Console.WriteLine(Convert.ToBoolean(1));
Console.WriteLine(Convert.ToBoolean(-1));
Console.WriteLine(Convert.ToBoolean(""));
Console.WriteLine(Convert.ToBoolean("LaunchCode"));

repl.it

5.1.3. Boolean Expressions

A boolean expression is an expression that evaluates to either True or False. The equality operator, ==, compares two values and returns true or false depending on whether the values are equal.

Example

Console.WriteLine(5 == 5);
Console.WriteLine(5 == 6);

Console Output

True
False

In the first statement, the two operands are equal, so the expression evaluates to True. In the second statement, 5 is not equal to 6, so we get False.

5.1.3.1. Comparison Operators

The == operator is one of six common comparison operators.

Comparison Operators

Operator	Description	Examples Returning True	Examples Returning False
Equal (==)	Returns True if the two operands are equal, and False otherwise.	7 == 7 "dog" == "dog"	7 == 5 "dog" == "cat"
Not equal(!=)	Returns True if the two operands are not equal, and False otherwise.	7 != 5 "dog" != "cat"	7 != 7 "dog" != "dog"
Greater than (>)	Returns True if the left-hand operand is greater than the right-hand operand, and False otherwise.	7 > 5 'b' > 'a'	5 > 7 'a' > 'b'
Less than (<)	Returns True if the left-hand operand is less than the right-hand operand, and False otherwise.	5 < 7 'a' < 'b'	7 < 5 'b' < 'a'
Greater than or equal (>=)	Returns True if the left-hand operand is greater than or equal to the right-hand operand, and False otherwise.	7 >= 5 7 >= 7 'b' >= 'a' 'b' >= 'b'	5 >= 7 'a' >= 'b'
Less than or equal (<=)	Returns True if the left-hand operand is less than or equal to the right-hand operand, and False otherwise.	5 <= 7 5 <= 5 'a' <= 'b' 'a' <= 'a'	7 <= 5 'b' <= 'a'

Although these operations are probably familiar, the C# symbols are different from the mathematical symbols. A common error is to use a single equal sign (=) instead of a double equal sign (==). Remember that = is an assignment operator and == is a comparison operator. Also note that =< and => are not recognized operators.

An equality test is symmetric, meaning that we can swap the places of the operands and the result is the same. For a variable a, if a == 7 is True then 7 == a is also True. However, an assignment statement is not symmetric: a = 7 is legal while 7 = a is not.

5.1.4. Check Your Understanding

Question

Under which conditions does Convert.ToBoolean() convert a string to True?

1. When the string is "true", "TRUE" or "True".

2. Whenever the string contains any non-whitespace character.

3. Whenever the string is non-empty.

4. Never. It converts all strings to False.

Question

Which of the following is a Boolean expression? Select all that apply.

1. 3 == 4

2. 3 + 4

3. 3 + 4 == 7

4. "false"