the combox is bound database table,but it is not displaying the contents of table in database,instead it is displaying corresponding null values(blank).it is showing just "select" and the blank spaces corresponding to the number of items in database

here is my code:

Function combobfill() As DataSet
addconnection.Open()
Dim selectstring As String = "select materialsname from materialtype_table"
Try
Dim da As New SqlDataAdapter(selectstring, addconnection)
Dim ds As New DataSet
ds.Tables.Add("mymaterial")
ds.Tables("mymaterial").Columns.Add("material")
ds.Tables("mymaterial").Rows.Add("Select")
da.Fill(ds, "mymaterial")
Return ds
Catch ex As Exception
MsgBox(ex.Message)
Finally
addconnection.Close()
End Try
Dim dsd As DataSet = Nothing
Return dsd

addconnection.Close()


End Function

Dim ds As DataSet = combofillobj.combobfill()
ComboBox_Mtype.DataSource = ds.Tables(0)
ComboBox_Mtype.DisplayMember = "material"