var UserID = (from row in dtp.AsEnumerable()
where row.Field<int>("UserID")==comboBox1.SelectedValue
select row.Field<int>("UserID")).FirstOrDefault();
</int></int>
Any error about AsEnumerable, just add this assembly:
C:\Windows\Microsoft.NET\Framework\v4.0.30319\System.Data.DataSetExtensions.dll