表“焊接工艺卡清单”中“所需焊工项目代号”列要生成多选目录树，内容来自表“焊工资质清单”中的列“焊工姓名”和列“考试合格项目代号”

窗口设计afterload事件

Dim cmd As New SQLCommand

Dim dt As DataTable

Dim cmb As WinForm.ComboBox

cmd.CommandText = "SELECT DISTINCT 焊工姓名,考试合格项目代号 Fro m {焊工资质清单}"

dt = cmd.ExecuteReader()

Dim trv As WinForm.TreeView = e.Form.Controls("TreeView1")

trv.BuildTree(dt,"焊工姓名|考试合格项目代号")

“确定”按钮click事件

Dim s As String = ""

Dim trv As WinForm.TreeView = e.Form.Controls("TreeView1")

For Each nd As WinForm.TreeNode In trv.AllNodes

    If nd.Checked Then

        s = s & "," & nd.Text

    End If

Next

e.Form.DropDownBox.Value = s.trim(",")

e.Form.DropDownBox.CloseDropdown

现在第一行勾选后，后面的行都会显示之前勾选的项目，能不能在勾选前自动清空，即不显示任何勾选项？