Фокус TextBox и ввод в него числа через кнопку

If TextBox1.Focus Then
            For i = 0 To TextBox1.Text.Length - 1
                If TextBox1.Text(i) = "," Then Exit Sub
            Next
            TextBox1.Text &= ","
        End If

If TextBox2.Focus Then
            For i = 0 To TextBox2.Text.Length - 1
                If TextBox2.Text(i) = "," Then Exit Sub
            Next
            TextBox2.Text &= ","
        End If

    Dim a As Byte = 0
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        If a = 1 Then
            For i = 0 To TextBox1.Text.Length - 1
                If TextBox1.Text(i) = "," Then Exit Sub
            Next
            TextBox1.Text &= ","
        ElseIf a = 2 Then
            For i = 0 To TextBox2.Text.Length - 1
                If TextBox2.Text(i) = "," Then Exit Sub
            Next
            TextBox2.Text &= ","
        End If
    End Sub
 
    Private Sub TextBox1_LostFocus(ByVal sender As Object, ByVal e As System.EventArgs) Handles TextBox1.LostFocus
        a = 1
    End Sub
 
    Private Sub TextBox2_LostFocus(ByVal sender As Object, ByVal e As System.EventArgs) Handles TextBox2.LostFocus
        a = 2
    End Sub