$host is a constant — and [uri].Host is exactly what you want to store in it
Building a full file URL from a server-relative path invites $host = ([uri]$web.Url).Host, and PowerShell answers "Cannot overwrite variable Host because it is read-only or constant" — an error about your variable name, phrased as if something were wrong with the value.
Skládání plné URL ze server-relativní cesty svádí napsat $host = ([uri]$web.Url).Host a PowerShell odpoví „Cannot overwrite variable Host because it is read-only or constant“ – chyba o názvu proměnné, formulovaná, jako by byl problém v hodnotě.
Symptom
A perfectly ordinary line in a SharePoint reporting script:
$web = Get-PnPWeb
$host = ([uri]$web.Url).GetLeftPart([System.UriPartial]::Authority)
$full = $host + $item['FileRef']Cannot overwrite variable Host because it is read-only or constant.The value is a plain string. The cast is fine. The error mentions neither reserved names nor automatic variables, so the first instinct is to inspect $web.Url.
Cause
$Host is an automatic variable holding the PowerShell host application, and it is declared Constant, AllScope — not merely read-only. Constant variables cannot be reassigned, cannot be removed, and cannot be shadowed by a local of the same name, which is why the error fires even inside a function.
The collision is not bad luck. Host is the natural name for the thing you get out of [uri], Uri.Host is literally the property name, and half of building SharePoint URLs is separating host from path.
Automatic variables are not uniformly protected — checked on 5.1 and 7:
| Variable | Options | Assignable |
|---|---|---|
$host |
Constant, AllScope |
no |
$error |
Constant |
no |
$PID, $PSHome, $true |
Constant, AllScope |
no |
$input |
None |
yes — and clobbering it breaks pipeline functions |
$args |
None |
yes — and clobbering it breaks parameter-less functions |
$profile |
None |
yes |
So the ones that bite loudly are the safe ones. $input and $args accept the assignment silently and break something subtle later.
Fix
Rename. Nothing else is available — the variable is a constant by design:
$authority = ([uri]$web.Url).GetLeftPart([System.UriPartial]::Authority)
$full = $authority + $item['FileRef']Check a name before you commit to it across a script:
(Get-Variable -Name host -ErrorAction SilentlyContinue).Options # Constant, AllScopeNotes
- PSScriptAnalyzer's
PSAvoidAssignmentToAutomaticVariableflags this statically, including the silent$input/$argscases that PowerShell itself never complains about. Worth having in the lint step for exactly those two. ${host}and$script:hostdo not get you around it; constant means constant in every scope.- Other names worth avoiding in SharePoint scripts for the same reason:
$matches(populated by every-matchyou run, so your value survives only until the next comparison) and$error(constant).