Invalid cross-thread access in Silverlight app

private void GetRequestStreamCallback(IAsyncResult asynchronousResult)
{
HttpWebRequest request = (HttpWebRequest)asynchronousResult.AsyncState;
Stream postStream = request.EndGetRequestStream(asynchronousResult);
ReqInvoiceHeader _ReqInvoice = new ReqInvoiceHeader();
_ReqInvoice.CardCode = strCmCode.Trim();
_ReqInvoice.CardName = strCmName.Trim();
_ReqInvoice.Remarks = srtRemarks.Trim();
ReqInvoiceLines oInvLine = new ReqInvoiceLines();
for (int i = 0; i < 2; i++)
{
//Hear is the Error
//Invalid cross-thread access
oInvLine.ItemCode = ((TextBlock)DataGridItem.Columns[0].GetCellContent(DataGridItem.SelectedItem)).Text;
oInvLine.Quantity = ((TextBlock)DataGridItem.Columns[1].GetCellContent(DataGridItem.SelectedItem)).Text;
oInvLine.UnitPrice = ((TextBlock)DataGridItem.Columns[2].GetCellContent(DataGridItem.SelectedItem)).Text;
 
}
_ReqInvoice.Details.Add(oInvLine);
MemoryStream ReqMs = new MemoryStream();
DataContractJsonSerializer jsondata = new DataContractJsonSerializer(typeof(ReqLogIn));
jsondata.WriteObject(ReqMs, _ReqInvoice);
ReqMs.Position = 0;
StreamReader sr = new StreamReader(ReqMs);
string postData = sr.ReadToEnd();
byte[] byteArray = Encoding.UTF8.GetBytes(postData);
postStream.Write(byteArray, 0, postData.Length);
postStream.Close();
request.BeginGetResponse(new AsyncCallback(GetResponseCallback), request);
}
ReqInvoiceLines cLASS
namespace Pos_Silverlight.DataContract
{
    [DataContract(Name = "ReqInvoiceLines")]
    public class ReqInvoiceLines
    {
        [DataMember(Name = "Product")]
        public string ItemCode { get; set; }
 
        [DataMember(Name = "Qty")]
        public string Quantity { get; set; }
 
        [DataMember(Name = "UnitPrice")]
        public string UnitPrice { get; set; }
        public ReqInvoiceLines()
        {
        }
        public ReqInvoiceLines(string Product, string Qty, string UnitPrice)
        {
            this.UnitPrice = UnitPrice;
            this.Quantity = Qty;
            this.ItemCode=ItemCode;
        }
    }
    public class ItemsCollection : Collection<ReqInvoiceLines>
    {
        public ItemsCollection()
        {
            Add(new ReqInvoiceLines("hb", "Item Name1", "1"));
            Add(new ReqInvoiceLines("del", "Item Name2", "2"));
            Add(new ReqInvoiceLines("con", "Item Name3", "3"));
 
        }
    }