PowerShell's -EQ Operator: Reference Equality vs Value Equality

A question recently came up asking why the -eq operator doesn't work for two different (but equal) SecureStrings.

 

By default, almost all environments (the .NET Framework included) test if two things are exactly the same – that they are stored in the same place in memory. This is called reference equality.

 

[D:\documents\WindowsPowerShell]

PS:14 > $test = Read-Host -AsSecureString

****

 

[D:\documents\WindowsPowerShell]

PS:15 > [Object]::ReferenceEquals($test, $test)

True

 

[D:\documents\WindowsPowerShell]

PS:16 > $test -eq $test

True

 

That’s not usually what people want, so each individual type of object is (optionally) responsible for supporting a value-based equality test. They do this by implementing interfaces (software contracts) called IComparable and / or IEquatable:

 

[D:\documents\WindowsPowerShell]

PS:17 > $string1 = "Test"

 

[D:\documents\WindowsPowerShell]

PS:18 > $string2 = "Test"

 

[D:\documents\WindowsPowerShell]

PS:19 > [Object]::ReferenceEquals($string1, $string2)

False

 

[D:\documents\WindowsPowerShell]

PS:20 > [String].GetInterfaces()

 

IsPublic IsSerial Name

-------- -------- ----

True     False    IComparable

True     False    ICloneable

True     False    IConvertible

True     False    IComparable`1

True     False    IEnumerable`1

True     False    IEnumerable

True     False    IEquatable`1

 

[D:\documents\WindowsPowerShell]

PS:21 > $string1 -eq $string2

True

 

 

The implementors of the SecureString class have chosen to not implement these contracts, so PowerShell supports only reference equality on these types:

 

[D:\documents\WindowsPowerShell]

PS:30 > [System.Security.SecureString].GetInterfaces()

 

IsPublic IsSerial Name

-------- -------- ----

True     False    IDisposable

 

[D:\documents\WindowsPowerShell]

PS:31 > $test1 = Read-Host -AsSecureString

****

 

[D:\documents\WindowsPowerShell]

PS:32 > $test2 = Read-Host -AsSecureString

****

 

[D:\documents\WindowsPowerShell]

PS:33 > $test1 -eq $test2

False

 

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