You cannot call a method in null expression

Question

I am just trying to create a powershell script that calculates the sum of the md5 executable (s).

My .ps1 script:

$answer = Read-Host "File name and extension (ie; file.exe)" $someFilePath = "C:\Users\xxx\Downloads\$answer" If (Test-Path $someFilePath){ $stream = [System.IO.File]::Open("$someFilePath",[System.IO.Filemode]::Open, [System.IO.FileAccess]::Read) $hash = [System.BitConverter]::ToString($md5.ComputeHash($stream)) $hash $stream.Close() } Else{ Write-Host "Sorry, file $answer doesn't seem to exist." } 

When running my script, I get the following error:

 You cannot call a method on a null-valued expression. At C:\Users\xxx\Downloads\md5sum.ps1:6 char:29 + $hash = [System.BitConverter]::ToString($md5.Compute ... + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + CategoryInfo : InvalidOperation: (:) [], RuntimeException + FullyQualifiedErrorId : InvokeMethodOnNull 

As far as I understand, this error means that the script is trying to do something, but the other part of the script does not have any information that allows the first part of the script to work correctly. In this case, $hash .

Get-ExecutionPolicy outputs Unrestricted .

What causes this error?
What is my null expression?

Any help is appreciated. I apologize if this is trivial and will continue my research.

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References:

http://blogs.technet.com/b/heyscriptingguy/archive/2013/03/27/troubleshoot-the-invokemethodonnull-error-with-powershell.aspx

How to get MD5 checksum in PowerShell

Answer 1

The simple answer for this is that you have an undeclared (null) variable. In this case, it is $md5 . From the comment you provided, it is necessary that this be indicated elsewhere in your code.

 $md5 = new-object -TypeName System.Security.Cryptography.MD5CryptoServiceProvider 

The error was that you are trying to execute a method that does not exist.

 PS C:\Users\Matt> $md5 | gm TypeName: System.Security.Cryptography.MD5CryptoServiceProvider Name MemberType Definition ---- ---------- ---------- Clear Method void Clear() ComputeHash Method byte[] ComputeHash(System.IO.Stream inputStream), byte[] ComputeHash(byte[] buffer), byte[] ComputeHash(byte[] buffer, int offset, ... 

.ComputeHash() of $md5.ComputeHash() was a null value. Entering into gibberish would create the same effect.

 PS C:\Users\Matt> $bagel.MakeMeABagel() You cannot call a method on a null-valued expression. At line:1 char:1 + $bagel.MakeMeABagel() + ~~~~~~~~~~~~~~~~~~~~~ + CategoryInfo : InvalidOperation: (:) [], RuntimeException + FullyQualifiedErrorId : InvokeMethodOnNull 

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PowerShell by default allows this to be executed as defined by StrictMode

If the Set-StrictMode parameter is disabled, it is assumed that uninitialized variables (version 1) have a value of 0 (zero) or $ zero, depending on the type. References to nonexistent properties return $ Null, and function syntax results that are invalid are modified with an error. Variables are not allowed without modification.