在绑定数据的基础上用代码添加一个"全部"选项，类似Excel筛选功能。下拉菜单中第一行是“全部”，单击“全部”能展开所有数据。在网了找了一整天了，也我有什么结果。还请各位大师帮忙，先谢谢了！

已解决，效果非常好。谢谢各位的关注！愿与大家分享。
        Dim MyConnection As New System.Data.OleDb.OleDbConnection(MySQLConnectionString)
        MyConnection.Open()
        Dim MySQL As String = "select distinct 主题 from 日记 ORDER BY 主题 DESC"
        Dim MyAdapter As New OleDbDataAdapter(MySQL, MyConnection)
        Dim MyDepartmentTable As New System.Data.DataTable()
        MyAdapter.Fill(MyDepartmentTable)
      
        Dim dr As DataRow = MyDepartmentTable.NewRow()
        dr("主题") = "(全选)"
        MyDepartmentTable.Rows.InsertAt(dr, 0)
      
        Me.主题ComboBox1.DataSource = MyDepartmentTable
        Me.主题ComboBox1.DisplayMember = "主题"
        Me.主题ComboBox1.ValueMember = "主题"
        If (MyConnection.State = ConnectionState.Open) Then
            MyConnection.Close()
        End If