Use a bool variabe stating if is your code who is changing the combo index,something like
bool codeChanging=false;
cmbregno_SelectedIndexChanged(sender...)
{
if (!codeChanging)
{
codeChanging=true;
cmbrollno= dr.GetValue(0).ToString())
}
else
{
codeChanging=false;
}
}
cmbrollno_SelectedIndexChanged(sender...)
{
if (!codeChanging)
{
codeChanging=true;
cmbregno= dr.GetValue(0).ToString())
}
else
{
codeChanging=false;
}
}