如何获取主键自增长最大值

用一个带返回值的方法获取自增长主键最大id,下面代码该如何该
public int getIdMax()
{
conn.Open();
string sql = "select max(tableId) from " + tableName;
SqlCommand cmd = new SqlCommand(sql, conn);
Int32 count = (Int32)cmd.ExecuteScalar();
conn.Close();
return count;
}

select IDENT_CURRENT('table_name')