This post is short but useful. We need just two lines (we can combine it to one):
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$allSites = Get-AzureRmWebApp -ResourceGroupName resource-group-name
@($allSites).GetEnumerator() | Restart-AzureRmWebApp
The trick is to use GetEnumerator() because above commandlet returns System.Collections.Generic.List instead of array of individual objects into pipeline.
This also works with Stop-AzureRmWebApp and Start-AzureRmWebApp.
Simple, fast and useful. What we need more :)