dataset itemarray to array

String.Join(separador, dr.ItemArray.Select(Function(x) x.ToString()).ToArray)

hashset to string separador

Dim enviar As New HashSet(Of String)
Dim str As String = ""

str = String.Join(@, enviar.ToArray)



fuente

Concatenar una lista y transformarla en un String

Dim lista As New List(Of String)
Dim str As String = String.Join("", lista.ToArray)