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about_Booleans

  • Artikkeli

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