基于組合框選定項填充datagridview，該組合框取決于資料庫中另一個選定的組合框

我正在創建一個添加學生視窗表單，它包含兩個組合框。第一個包含學生的學期，第二個包含基于學期值的課程代碼。我面臨的問題是我也有一個datagridview，我想用課程代碼表中包含的學生資料填充它，只要我點擊課程代碼組合框更改的學期組合框值，只要它改變，我想加載 datagridview 的資料，但它顯示的錯誤基本上是無法將資料行轉換為字串，但是當我使用 ComboBox1.ValueMember 并在單擊按鈕上加載 datagridview 時，它作業正常。但是當我在組合框更改上加載 datagridview 時，它顯示錯誤。

此代碼作業正常

Imports System.Data.OleDb
Public Class AddStudent
    Dim con As New OleDbConnection("Provider = Microsoft.ACE.OLEDB.12.0;Data Source = C:\Users\Students.accdb")
    Dim con1 As New OleDbConnection("Provider = Microsoft.ACE.OLEDB.12.0;Data Source = C:\Users\Users.accdb")
    Private Sub AddStudent_Load(sender As Object, e As EventArgs) Handles MyBase.Load

    End Sub

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        con.Open()
        Dim cmd As New OleDbCommand("Select stud_roll From "   ComboBox1.SelectedValue   " where stud_roll = @roll1", con)
        cmd.Parameters.AddWithValue("roll1", TextBox2.Text)
        Dim myreader As OleDbDataReader = cmd.ExecuteReader
        If myreader.Read() Then
            con.Close()
            MessageBox.Show("Student Inserted before")
        Else
            con.Close()
            Dim cmd1 As New OleDbCommand("Insert into "   ComboBox1.SelectedValue   "(stud_roll,stud_name,date_of_birth,course,gender,mobile_no,semester) Values(@roll,@name,@dob,@course,@gender,@mobile,@semester)", con)
            cmd1.Parameters.AddWithValue("roll", Convert.ToInt32(TextBox2.Text))
            cmd1.Parameters.AddWithValue("name", TextBox1.Text)
            cmd1.Parameters.AddWithValue("dob", DateTimePicker1.Value.Date)
            cmd1.Parameters.AddWithValue("course", ComboBox1.SelectedValue)
            cmd1.Parameters.AddWithValue("gender", ComboBox2.SelectedItem)
            cmd1.Parameters.AddWithValue("mobile_no", MaskedTextBox1.Text)
            cmd1.Parameters.AddWithValue("semester", Convert.ToInt32(ComboBox3.SelectedItem))
            con.Open()
            cmd1.ExecuteNonQuery()
            con.Close()
            MessageBox.Show("Records inserted successfully")
        End If
        con.Open()
        Dim cmd3 As New OleDbCommand("Select stud_roll, stud_name, date_of_birth, course, gender,mobile_no, semester From "   ComboBox1.SelectedValue   "", con)
        Dim da As New OleDbDataAdapter
        da.SelectCommand = cmd3
        Dim dt As New DataTable
        dt.Clear()
        da.Fill(dt)
        DataGridView1.DataSource = dt
        con.Close()
    End Sub

    Private Sub Label8_Click(sender As Object, e As EventArgs) Handles Label8.Click
        AddTeacher.Show()
    End Sub

    Private Sub ComboBox3_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ComboBox3.SelectedIndexChanged
        con1.Open()
        Dim cmd As New OleDbCommand("Select course_code From courses Where semester= "   ComboBox3.SelectedItem   "", con1)
        Dim da As New OleDbDataAdapter
        da.SelectCommand = cmd
        Dim dt As New DataTable
        dt.Clear()
        da.Fill(dt)
        ComboBox1.DataSource = dt
        ComboBox1.DisplayMember = "course_code"
        ComboBox1.ValueMember = "course_code"
        con1.Close()
    End Sub

    Private Sub Label14_Click(sender As Object, e As EventArgs) Handles Label14.Click
        Courses.Show()
    End Sub

    Private Sub ComboBox1_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ComboBox1.SelectedIndexChanged

    End Sub
End Class

當我像這樣更改組合框后加載時，它顯示錯誤：- System.InvalidCastException: 'Operator ' ' is not defined for string "Select stud_name, stud_roll From" 并輸入 'DataRowView'。這是因為 datarowview 型別轉換

Private Sub ComboBox1_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ComboBox1.SelectedIndexChanged
    con.Open()
    Dim cmd3 As New OleDbCommand("Select stud_roll, stud_name, date_of_birth, course, gender,mobile_no, semester From "   ComboBox1.SelectedValue   "", con)
    Dim da As New OleDbDataAdapter
    da.SelectCommand = cmd3
    Dim dt As New DataTable
    dt.Clear()
    da.Fill(dt)
    DataGridView1.DataSource = dt
    con.Close()
End Sub 

請幫我解決一下這個。

uj5u.com熱心網友回復：

1.確保給控制元件起有意義的名字：它會讓你的生活更輕松。“ComboBox1”不是一個有意義的名稱；）

2.不要創建重復使用的資料庫連接的單個實體：您應該創建連接物件，使用它，然后處理它。幕后有一些機制可以提高效率（連接池）。在我以前的答案中有一個如何做到這一點的例子。

3.您不需要打開和關閉連接da.Fill(dt)：它會自動完成。

4.因為ComboBox1.SelectedValue來自資料表，您需要提取該資料行視圖的列，例如：

Dim tableName = DirectCast(ComboBox1.SelectedItem, DataRowView).Row.Field(Of String)("course_code")
Dim cmd3 As New OleDbCommand("Select stud_roll, stud_name, date_of_birth, course, gender,mobile_no, semester From [" & tableName & "]", con)

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