从Dropdownlist中选择后更改文本框值

Question

当我从第一个下拉列表中选择一个值时，它会将第二个下拉列表和文本框填充到正确的值。但是，当我从第二个下拉列表中选择不同的值时，文本框值仍保留与填充它的值相同的值。谁能指出我做错了什么？

 If ds.Tables(0).Rows(0).Item(3).ToString = "M" Then
            DropDownList2.Items.Insert(0, "Member")
            DropDownList2.Items.Insert(1, "Non-Member")
            DropDownList2.Items.Insert(2, "")
            TextBox1.Text = "M"
        ElseIf ds.Tables(0).Rows(0).Item(3).ToString = "NM" Then
            DropDownList2.Items.Insert(0, "Non-Member")
            DropDownList2.Items.Insert(1, "Member")
            DropDownList2.Items.Insert(2, "")
            TextBox1.Text = "NM"
        Else
            DropDownList2.Items.Insert(0, "")
            DropDownList2.Items.Insert(1, "Member")
            DropDownList2.Items.Insert(2, "Non-Member")
            TextBox1.Text = ""
        End If



        If DropDownList2.SelectedValue = "Member" Then
            TextBox1.Text = ""
            TextBox1.Text = "M"
        ElseIf DropDownList2.SelectedValue = "Non-Member" Then
            TextBox1.Text = ""
            TextBox1.Text = "NM"
        Else
            TextBox1.Text = ""
 End If