...
cmd.Parameters.Add(new SqlParameter("@aa", SqlDbType.Int));
cmd.Parameters["@aa"].Value = aaValue;
cmd.Parameters.Add(new SqlParameter("@bb", SqlDbType.Int));
cmd.Parameters["@bb"].Value = bbValue?? (object)DBNull.Value;
reader = cmd.ExecuteReader(); ...

SELECT ID
FROM MyTable
WHERE aa = @aa AND (bb IS NULL OR bb = @bb)

SELECT ID  from MyTable where  aa = @aa and (@bb is null OR bb = @bb)