Hi Try this
AutoCompleteStringCollection AutoItem = new AutoCompleteStringCollection();
foreach (DataRow OSAmt in dataSetStockistOutstanding1.OutstandingAmount1)
{
AutoItem.Add(OSAmt["CustCode"].ToString());
AutoItem.Add(OSAmt["CustCompName"].ToString());
AutoItem.Add(OSAmt["ChequeNo"].ToString());
AutoItem.Add(OSAmt["InvoiceNo"].ToString());
}
Tesearchupdate.AutoCompleteMode = AutoCompleteMode.Suggest;
Tesearchupdate.AutoCompleteSource = AutoCompleteSource.CustomSource;
Tesearchupdate.AutoCompleteCustomSource = AutoItem;