A:
try
{
    i = (int)sqlcommand1.ExecuteScalar(selectstring);
}
catch
{
}
B:
if(sqlcommand1.ExecuteScalar(selectstring) != null)
{
   i = (int)sqlcommand1.ExecuteScalar(selectstring);
}
在sqlcommand1.ExecuteScalar(selectstring)为空和不为空的情况下综合考虑.
谢谢大家帮忙!