ABOUT_BOOLEANS


Short description

Describes how boolean expressions are evaluated.


Long description

PowerShell can implicitly treat any type as a BOOLEAN. It is important to
understand the rules that PowerShell uses to convert other types to BOOLEAN
values.


Converting from scalar types

A scalar type is an atomic quantity that can hold only one value at a time.
The following types evaluate to $false:

-   Empty strings like '' or ""
-   Null values like $null
-   Any numeric type with the value of 0

Examples:

    PS> $false -eq ''
    True
    PS> if ("") { $true } else { $false }
    False
    PS> if ($null) { $true } else { $false }
    False
    PS> if ([int]0) { $true } else { $false }
    False
    PS> if ([double]0.0) { $true } else { $false }
    False

The following types evaluate to $true:

-   Non-empty strings
-   Instances of any other non-collection type

Examples:

    # a non-collection type
    PS> [bool]@{value = 0}
    True
    # non-empty strings
    PS> if ('hello') { $true } else { $false }
    True
    PS> [bool]'False'
    True

Note that this differs from _explicit string parsing_:

    PS> [bool]::Parse('false')
    False
    PS> [bool]::Parse('True')
    True
    PS> [bool]::Parse('Not True')
    MethodInvocationException: Exception calling "Parse" with "1" argument(s):
    "String 'Not True' was not recognized as a valid Boolean."


Converting from collection types

Arrays are the most common collection type in PowerShell. These rules apply
to any collection-like types that implement the IList interface.

-   Empty collections are always $false
-   The special null value indicating the absence of output from a command,
    [System.Management.Automation.Internal.AutomationNull]::Value is always
    $false.
-   Single-element collections evaluate to the BOOLEAN value of their one
    and only element.
-   Collections with more than 1 element are always $true.

Examples:

    # Empty collections
    PS> [bool]@()
    False
    PS> [bool](Get-ChildItem | Where-Object Name -eq 'Non-existent-File.txt')
    False
    # Single-element collections
    PS> $a = @(0)
    PS> [bool]$a
    False
    PS> $b = @(1)
    PS> [bool]$b
    True
    # Multi-element collections
    PS> $c = @(0,0)
    PS> [bool]$c
    True


See also

-   about_Arrays