The SqlParameterCollection only accepts non-null SqlParameter type objects, not String objects

Question

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    Dim ds As DataSet = New DataSet()
    Dim param As SqlParameter = New SqlParameter("@user_code", SqlDbType.Char, 4)
    param.Value = "0016"
    Const sqlstr As String = "select sectors.sector_code,sector_name   from user_sectors inner join sectors on user_sectors.sector_code = sectors.sector_code  where user_code = @user_code  and sectors.sector_code not in ('z')"
    Dim da As SqlDataAdapter = New SqlDataAdapter(sqlstr, _con)
    da.SelectCommand.Parameters.Add("@user_code")
    da.Fill(ds)
    ddl.DataValueField = "sector_code"
    ddl.DataTextField = "sector_name"
    ddl.DataSource = ds.Tables(0)
    ddl.DataBind()    
End Sub

Answer 1

I guess line

da.SelectCommand.Parameters.Add("@user_code")

should be

da.SelectCommand.Parameters.Add(param)