List<List1Dto> listResult = new List<List1Dto>();            

listResult.Add(new List1Dto { UserName = "user1", UserPwd = "user1", UserTitle = "user1", OrderNumber = "1" });            

listResult.Add(new List1Dto { UserName = "user2", UserPwd = "user2", UserTitle = "user2", OrderNumber = "2" });            

listResult.Add(new List1Dto { UserName = "user10", UserPwd = "user10", UserTitle = "user10", OrderNumber = "10" });            

listResult.Add(new List1Dto { UserName = "user20", UserPwd = "user20", UserTitle = "user20", OrderNumber = "20" });

List<List1Dto> list1 = listResult.OrderBy(ao => ao.OrderNumber).ToList();            

string tmp = "";            

foreach (List1Dto item in list1)            

{                

　　tmp += item.UserName + "\r\n";           

}

结果:

user1

user10

user2

user20

修改:

List<List1Dto> list1 = listResult.OrderBy(ao => Convert.ToInt32( string.IsNullOrEmpty(ao.OrderNumber) ? "0" : ao.OrderNumber)).ToList();