Sql Statements With Null Search Parameters

i have a form where filter information is going to be keyed/selected.
the data from that form is copied into a struct regardless of whether each field has data in it or not (eg firstname may have been entered but not lastname,
so the struct would contain firstname = 'john' and lastname = null)
i then want to build an sql statement (not using stored procedures but just direct sql) from this struct data but obvioulsy can't just have
a statement that says 'select ID from theTable where firstname='john' AND lastname=''" as this won't bring back the right info, if anything at all.

i normally build the statements like this




Code Block
string sql = "select ID from MembersTemp where FirstName = '{0}'";
SqlCommand cmd = new SqlCommand(string.Format(sql, FirstName), _con);
_con.Open();
SqlDataReader rdr = cmd.ExecuteReader();
List<int> result = new List<int>();
while (rdr.Read())
{
etc....






so really, what's the best way of building an sql statement where some search parameters won't hold a value and therefore need ignoring.


also, whilst i'm here, i use viz studio 2005 and sql server 2005 (express??), so that does mean i'm using TSQL as well??