选项严格打开，设置未知对象类型的焦点

Question

我正在更新一些代码以允许Option Strict On。即将出现的一个问题是后期约束。我有一个Form，其中包含多个不同类型的必需项目（TextBox，ComboBox等等。）我有一个功能来检查Form的有效性，以及然后将焦点设置为第一个没有值的控件。

如果没有Option Strict On，我可以简单地使用基本Object，并将其设置为缺少值的控件，然后在结尾处调用objMissing.Focus()，但选项Strict On，编译器不允许后期绑定。

我意识到如果控件都是相同的类型，我可以将丢失的对象转换为TextBox。有没有办法我仍然可以使用一个变量存储控件来设置焦点？或者我应该立即在每个正在检查值的If中设置焦点吗？

以下是我正在查看的代码示例（txt_为TextBox，cbo_为ComboBox，btn_为Button类型）：

    Dim objMissing as Object

    If txtItemDescription.Text = String.Empty Then
        objMissing = txtItemDescription
        strMessage = "You must enter an item description."
    ElseIf cboProductType.Text = String.Empty Then
        objMissing = cboProductType
        strMessage = "You must select a product type."
    ElseIf cboComponentType.Text = String.Empty And cboComponentType.Enabled Then
        objMissing = cboComponentType
        strMessage = "You must select a component type."
    ElseIf txtOnHand.Text = String.Empty Then
        txtOnHand.Text = "0"
    ElseIf txtRented.Text = String.Empty Then
        txtRented.Text = "0"
    ElseIf txtCost.Text = String.Empty Then
        txtCost.Text = "0.00"
    ElseIf txtFreight.Text = String.Empty Then
        txtFreight.Text = "0.00"
    ElseIf Len(txtBarcodePrefix.Text) < 6 Then
        objMissing = txtBarcodePrefix
        strMessage = "You must enter a 6-digit barcode prefix."
    ElseIf cboCondition.Text = String.Empty Then
        objMissing = cboCondition
        strMessage = "You must enter a condition."
    ElseIf btnComponents.Enabled And Me.ComponentList.Count = 0 Then
        objMissing = btnComponents
        strMessage = "You must select the item components."
    ElseIf txtSerialNumber.Text <> String.Empty AndAlso txtOnHand.Text <> String.Empty Then
        If CInt(txtOnHand.Text) > 1 Then
            objMissing = txtOnHand
            strMessage = "You cannot have more than 1 item on hand with the same serial number."
        End If
    End If

    If objMissing IsNot Nothing Then
        MessageBox.Show(strMessage)
        objMissing.Focus()
    End If

Answer 1

如果将objMissing声明为类型Control，那么您的代码将按您的意愿运行。所有标准的WinForms控件都应该从Control继承。