Class MyClass {
      
    [int] Add47([int] $value) {    
      return 47 + $value
    }
            
    [int] DoStuff([System.Collections.Generic.IEnumerable[int]] $collection) {
        # ugly
        $func = $this.GetType().GetMethod("Add47").CreateDelegate([Func[int,int]], $this)
        return [Linq.Enumerable]::Sum($collection, $func)
    }
}

Class MyClass {    
    [int] Add47([int] $value) {    
      return 47 + $value
    }
            
    [int] DoStuff([System.Collections.Generic.IEnumerable[int]] $collection) {
        return [Linq.Enumerable]::Sum($collection, $this.Add47)
    }
}