adding two items into DataGridComboBoxColumn

Public Property Warehouse() As ObservableCollection(Of String)
Function loadWarehouse()
Dim a As String
Dim dbConn As System.Data.SqlClient.SqlConnection = New System.Data.SqlClient.SqlConnection(ConfigurationManager.AppSettings("ConnectionString"))
Dim ConString As String = ConfigurationManager.ConnectionStrings("CompuTrakwpf.MySettings.CompuTrakDBConnectionString").ConnectionString
dbConn.ConnectionString = ConString : dbConn.Open()
Dim DSet As New DataSet, SQLStr As String
Dim cmd As System.Data.SqlClient.SqlCommand
Dim dbAdaptr As System.Data.SqlClient.SqlDataAdapter = New System.Data.SqlClient.SqlDataAdapter
Dim tRow As DataRow, tTbl As DataTable
Try
With dbAdaptr
.TableMappings.Add("Table", "Warehouse")
SQLStr = "Select [Warehouse] from [Warehouse] order by [Warehouse]"
cmd = New System.Data.SqlClient.SqlCommand(SQLStr, dbConn)
cmd.CommandType = CommandType.Text
.SelectCommand = cmd
.Fill(DSet)
.Dispose()
End With
DSet.AcceptChanges()
tTbl = DSet.Tables.Item(0)
DSet.Dispose() : dbConn.Close()
Warehouse = New ObservableCollection(Of String)() From {""}
For Each tRow In tTbl.Rows
a = (tRow("Warehouse").ToString)
Warehouse.Add(a)
Next
Return 0
Catch ex As Exception
MsgBox(Err.Description)
End Try
End Function


<DataGridComboBoxColumn x:Name="cboWarehouse" Header="Warehouse" Width="70" SelectedItemBinding="{Binding WareHouse}"/>