'Angenommene Datentypen der Variablen
    Dim gp_bezeichnung As New TextBox
    Dim gp_name As TextBox
    Dim partnerliste As New ListView
    Dim label2 As New Label
 
 
 
    Private Sub Button3_Click(sender As Object, e As EventArgs) Handles ...
 
        Dim GP As String = ""
        If Not String.IsNullOrWhiteSpace(gp_bezeichnung.Text) Then
            GP = gp_bezeichnung.Text
        ElseIf Not String.IsNullOrWhiteSpace(gp_name.Text) Then
            GP = gp_name.Text
        End If
 
        Me.label2.text = "nichts"
 
        If Not String.IsNullorWhiteSpace(GP) then
            With Me.partnerliste
              For i As Integer = 0 To .CheckedItems.Count - 1
                 'Vergleich CaseInsensitiv?  Evt. StringComparison nutzen? 
                 If .CheckedItems(i).SubItems(0).Text.ToLower = GP.ToLower Then
                    Me.label2.Text = "Ja"
                 ElseIf Me.label2.Text <> "Ja" Then
                    Me.label2.Text = "Nein"
                 End If
             Next i
           End With
        End If
    End Sub