我创建了一个表单，将销售数据保存到数据库中，如果用户没有填写所有字段，它应该显示一条错误消息或只突出显示空文本框。我使文本框突出显示，如果空的，但一些文本框，如日期和两个组合框没有突出显示。如果文本框不为空，则保存数据，我已经编写了将数据保存到数据库的代码。我只是不知道如何构造else语句，使其在文本框不为空时保存数据。屏幕截图：https://snipboard.io/t5keMo.jpg
这是我的代码：

Public Sub btnAdd_Click(sender As Object, e As EventArgs) Handles btnAdd.Click

    Dim txt As Control
    Dim combo As New Control

    '  While 
    For Each txt In Panel1.Controls
        If TypeOf txt Is TextBox Then
            If txt.Text = "" Then
                txt.BackColor = Color.Yellow
                txt.Focus()
            End If
        End If
    Next
    For Each combo In Panel1.Controls
        If TypeOf combo Is ComboBox Then
            If combo.Text = "" And combo.Text = "" Then
                combo.BackColor = Color.Yellow
                combo.Focus()
            End If
        End If
    Next
    For Each Dt In Panel1.Controls
        If TypeOf Dt Is Date Then
            If Dt.Text = "" Then
                Dt.BackColor = Color.Yellow
                Dt.Focus()
            End If
        End If
    Next
    ' End While

End Sub

保存数据：

Public Sub save_data()
    ' load_table()
    MysqlConn = New MySqlConnection
    MysqlConn.ConnectionString = "server=localhost;userid=root;password=root;database=golden_star"
    Dim READER As MySqlDataReader


    Try
        MysqlConn.Open()
        Dim Query As String

        If (MessageBox.Show("Do you want to save the changes?", "Save Changes", MessageBoxButtons.YesNo, MessageBoxIcon.Question) = DialogResult.Yes) Then
            Query = "insert into golden_star.sales (date,brand,size,selling_unit_price,cost_unit_price,quantity,cost_of_goods,profit,total_cost_price) values ('" & DateTimePicker.Text & "','" & ComboBox1.Text & "', '" & ComboBox2.Text & "', '" & txtSelling.Text & "', '" & txtCostPrice.Text & "', '" & ComboBox3.Text & "', '" & txtTotalSell.Text & "', '" & txtProfit.Text & "',
        '" & txtCp.Text & "')"

            Command = New MySqlCommand(Query, MysqlConn)
        MessageBox.Show("Daily Sales Entered Successfully")
        READER = Command.ExecuteReader
        MysqlConn.Close()
        End If
    Catch ex As MySqlException
        MessageBox.Show(ex.Message)

    Finally
        MysqlConn.Dispose()
    End Try
End Sub