How to execute powershell code which is in variable

A similar thing is fully documented in the MS Docs PowerShell help file.

https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.utility/invoke-expression?view=powershell-7

You will note that at no point is the formatted multi-line code used. It's all one-liner execution code.

Note that in the call of your code, it's just this.

    @($myvar = "i am here"
    if ($myvar -ne $null) {
        "($myvar) variable is Full"
    } else {
        "($myvar) variable is Empty"
    }) | 
    Out-File -FilePath 'D:\Temp\MyCode.ps1'

    ($Command = Get-Content -Path 'D:\Temp\MyCode.ps1')

    # Results the code just runs with no Invoke-* needed
    <#
    (i am here) variable is Full
    #>

Update as per my comments

You define a var, with whatever string you want, and just type the var name. But you must do it all in the PowerShell process.

Why define PowerShell code outside the PowerShell process, just to send it to the PowerShell process?

# In a PowerShell Script development session

$MyCode = @($myvar = "i am here"
if ($myvar -ne $null) {
    "($myvar) variable is Full"
} else {
    "($myvar) variable is Empty"
})

$MyCode

# Results
<#
(i am here) variable is Full
#>

$myvar = "i am here"
if ($myvar -ne $null) {
    "($myvar) variable is Full"
} else {
    "($myvar) variable is Empty"
}

$myvar
# Results
<#
i am here
#>

# In a PowerShell terminal interactive session. 

$myvar = "i am here";if ($myvar -ne $null) {"($myvar) variable is Full"} else {"($myvar) variable is Empty"}

$myvar
# Results
<#
i am here
#>

$myvar = $null;if ($myvar -ne $null) {"$myvar variable is Full"} else {Write-Warning -Message "$myvar variable is Empty"}

# Results
<#
WARNING:  variable is Empty
#>

You cannot define a var outside of PowerShell then start PowerShell and try to use that var content. It's not in scope. All code, vars, functions, etc., have a scope and the scope must be defined in the PowerShell process

If you are saying you are wanting to run PowerShell code started from another terminal, say cmd.exe, then PowerShell has start parameters, and proper quoting, when needed, is a thing.

# There are several PowerShell startup parameters
<#
The syntax of Powershell.exe Command is:

PowerShell[.exe] 
[-EncodedCommand ]
[-ExecutionPolicy ]
[-InputFormat {Text | XML}]
[-Mta]
[-NoExit]
[-NoLogo]
[-NonInteractive]
[-NoProfile]
[-OutputFormat {Text | XML}]
[-PSConsoleFile ]
[ -Version <Windows PowerShell version> ]
[-Sta]
[-WindowStyle ]<style>
[-File <FilePath> [<Args>]]
[-Command { - | <script-block>  [-args <arg-array> ] | <string>  [<CommandParameters>] } ]
#>
powershell -Command  "Whatever command you want to run"

# Example, from a cmd.exe prompt
powershell -NoProfile -Command Get-Date
# Results
<#
Sunday, 14 June, 2020 04:31:38
#>

powershell -NoProfile -Command '{"Hello World"}'
# Results
<#
Hello World
#>

You cannot do this ...

${yourcode} | powershell

... outside of a PowerShell session. The PowerShell pipeline is only available in a PowerShell session. If you are already in a session, then again, just type and run the code.

Whatever terminal you are in has to be able to send this to PowerShell to run via the normal PowerShell startup. Yet, again, this is just introducing unneeded complexity, vs just running the script of running the code in a PowerShell interactive session directly.

You could store the code from the variable in a temporary script file and then execute it:

$code_in_var = cat .\file1.ps1

$tmpScriptFile = New-TemporaryFile | Rename-Item -NewName {$_.Name -ireplace 'tmp$', 'ps1'} -PassThru
Set-Content -Path $tmpScriptFile -Value $code_in_var
& $tmpScriptFile
Remove-Item -Path $tmpScriptFile

But if the code already originates from a file, just directly execute that instead of course.

This has been killing me, and that is how I ended up looking at this question. I know it is old. And I have a use case much like yours in my work. I don't want to leave my scripts on random client systems. I need to do more testing, but I just found that this works for a store Script in a Variable. Sorry, I don't know how to do the code formatting here. Hope this helps.

$myScript = (get-content .\\Helloworld.ps1)

$myScript | out-string | invoke-expression

In my case, I want to get $myScript, which will be a catted file from a remote server through SSH.