If you are reading a single row and a single column: just use ExecuteScalar()
:
string rs = (string)cmd.ExecuteScalar();
But to answer your question, the normal usage is:
using(var reader = cmd.ExecuteReader())
{
while(reader.Read())
{
// read a row, for example:
string foo = reader.GetString(0);
Console.WriteLine(foo);
}
}