Why is $false -eq "" true?

Question

The following code segments output true:

$x = ($false -eq "") 
Write-Host $x

$x = ($false -eq 0) 
Write-Host $x

Since $false and "" are different data types, shouldn't it automatically equal false?

Answer 1

When doing comparison operations, PowerShell will automatically attempt to coerce the object on the right-hand side of the operator to match the type on the left-hand side.

In the case of coercing [string] to [bool], any non-null string will evaluate as $true, and a null string will evaluate as $false. See blog post Boolean Values and Operators for more information about automatic conversion of different data types to boolean values.

This sometimes leads to unexpected results:

PS C:\> [bool]"$false" 

True

The string value of $false is 'False', which is a non-null string and evaluated to $true when cast back to [bool].

It also makes comparison operations non-commutative when the operands are of different data types:

PS C:\> '' -eq $false
False
PS C:\> $false -eq ''
True

In the first comparison the value $false is auto-cast to a string in order to match the type of the first operand (''), so you're actually comparing '' -eq 'False', which evaluates to $false.

In the second comparison the string '' is auto-cast to a boolean, again in order to match the type of the first operand ($false), so this time you're actually comparing $false -eq $false, which evaluates to $true.

Answer 2

Just sharing one experience here which might be worth noting when we convert a string value to boolean:

What I was doing is reading a boolean string value from a configuration file which was getting stored in a variable as shown below:

$valueReadFromFile = "false"

Now, I wanted to convert it to Boolean value. Since I was not aware of Convert class in PowerShell I used casting instead as shown below in a bool condition of if block:

if([bool]$valueReadFromFile -eq $true)
{
    echo "This message shouldn't get printed in current scenario"
}

But I was on a wrong turn. Everytime below message was getting printed because non-empty string in PowerShell gets casted to literal boolean value $true:

This message shouldn't get printed in current scenario

When my program started to behave incorrectly then I explored more and came to know about convert class. I fixed my code as below:

$actualBoolValue = [System.Convert]::ToBoolean($valueReadFromFile.Trim())