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bool (C# Reference)

The bool keyword is an alias of System.Boolean. It is used to declare variables to store the Boolean values, true and false.

NoteNote

If you require a Boolean variable that can also have a value of null, use bool. For more information, see Nullable Types (C# Programming Guide).

You can assign a Boolean value to a bool variable. You can also assign an expression that evaluates to bool to a bool variable.

// keyword_bool.cs
using System;
public class MyClass
{
    static void Main()
    {
        bool i = true;
        char c = '0';
        Console.WriteLine(i);
        i = false;
        Console.WriteLine(i);

        bool Alphabetic = (c > 64 && c < 123);
        Console.WriteLine(Alphabetic);
    }
}

Output

 
True
False
False

In C++, a value of type bool can be converted to a value of type int; in other words, false is equivalent to zero and true is equivalent to nonzero values. In C#, there is no conversion between the bool type and other types. For example, the following if statement is invalid in C#, while it is legal in C++:

int x = 123;
if (x)   // Invalid in C#
{
    printf_s("The value of x is nonzero.");
}

To test a variable of the type int, you have to explicitly compare it to a value, such as zero, as follows:

int x = 123;
if (x != 0)   // The C# way
{
    Console.Write("The value of x is nonzero.");
}

In this example, you enter a character from the keyboard and the program checks if the input character is a letter. If so, it checks if it is lowercase or uppercase. These checks are performed with the IsLetter, and IsLower, both of which return the bool type:

// keyword_bool_2.cs
using System;
public class BoolTest 
{
    static void Main() 
    {
        Console.Write("Enter a character: "); 
        char c = (char)Console.Read();
        if (Char.IsLetter(c))
        {
            if (Char.IsLower(c))
            {
                Console.WriteLine("The character is lowercase.");
            }
            else
            {
                Console.WriteLine("The character is uppercase.");
            }
        }
        else
        {
            Console.WriteLine("Not an alphabetic character.");
        }
    }
}

Input

 
X

Sample Output

 
Enter a character: X
The character is uppercase.
Additional sample runs might look as follow:
Enter a character: x
The character is lowercase.

Enter a character: 2
The character is not an alphabetic character.

For more information on bool and related subjects, see the following sections in the C# Language Specification:

  • 4.1.8 The bool Type

  • 7.9.4 Boolean Equality Operators

  • 7.11.1 Boolean Conditional Logical Operators

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