Public Class Form1
    Dim cur_operator As String = ""
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button1.Click
        If cur_operator = "" Then
            TextBox1.AppendText("1")
        Else
            TextBox2.AppendText("1")
        End If
    End Sub
 
    Private Sub Plus_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Plus.Click
        cur_operator = "+"
    End Sub
End Class