Public Class Form1

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

        Dim n As Integer
        Dim j As Integer
        Dim sum As Integer
        n = TextBox1.Text

        Do While n > 0

            j = n Mod 10
            sum += j
            n /= 10

        Loop
        MsgBox(sum)
    End Sub
End Class