如何在asp.net中给出Null值条件

Question

我希望在（Me.txt_rest_value_assessment.Tex）= NULL时此条件不起作用。因为在这种情况下，它会给出错误“从double转换为字符串错误”

If (Me.txt_rest_value_assessment.Tex) = NULL，然后它应该绕过这个条件

If CDbl(Me.txt_rest_value_assessment.Text) > _
   (CDbl(Me.txt_business_revenue_risk.Text) + CDbl(Me.txt_project_upside_value.Text)) Then

    Me.pnl_edit.Visible = True
    Me.lbl_conf_message.Text = "Rest Value Assessment value cannot be greater than Sum of Project Outlook Value and Project Upside Value."

End If