在文本框中使用math.round off函数时出错

Question

如果选中了Roundoff textbox1，我想要对2个文本框中的值进行舍入，例如textbox2和menustrip，如果未选中则保留值。 textbox1和textbox2包含一些计算结果，该结果将始终为数字。我使用以下代码但在调试"Conversion from string "" to type 'Double' is not valid."

时发生错误

注意：文本框的ReadOnly属性设置为True

Private Sub mnuRoundOff_CheckedChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles mnuRoundOff.CheckedChanged
        Dim r, s As String
        Dim t, u As Decimal
        r = TextBox2.Text
        s = TextBox3.Text
        t = r * 1
        u = s * 1
        If mnuRoundOff.Checked = True Then
            TextBox2.Text = Math.Round(t, 0, MidpointRounding.AwayFromZero)
            TextBox3.Text = Math.Round(u, 0, MidpointRounding.AwayFromZero)
        Else
            TextBox2.Text = r
            TextBox3.Text = s
        End If
    End Sub