如何增加字符串中的数字？

Question

I want to increment a number in a string.

My model:

public partial class updown
{
    public int id { get; set; }
    public string number { get; set; }
}

My controller:

[HttpPost]
public ActionResult Index(updown viewModel)
{
    var lastPO = db.updowns
                   .OrderByDescending(o => o.number)
                   .Select(m => m.number)
                   .FirstOrDefault() ?? "00001";

    item.number = lastPO++; // I get an error on this line
    db.updowns.Add(item);
    db.SaveChanges();

    return View(viewModel);
}

The error:

Operator '++' cannot be applied to operand of type 'string'

Expected result:

item.number // "00002"

Update

Based on the now-deleted comments, I have:

[HttpPost]
public ActionResult Index(updown viewModel)
{
    var s = db.updowns
              .OrderByDescending(o => o.number)
              .Select(m => m.number)
              .FirstOrDefault() ?? "00001/S2";

    var lastPO = Convert.ToInt32(s.Substring(s.Length - 5 - 3, s.Length - 3)); // *
    item.number = "EMOC-" + (++lastPO).ToString("00000");
    db.updowns.Add(item);
    db.SaveChanges();

    return View(viewModel);
}

Expected result:

item.number // "EMOC-00002/S2"

But I get an error on the line marked // * when I run it the second time.

Answer 1

I tried doing a little bit of your code. It seems fine

i think the problem lies in here

var s = db.updowns
              .OrderByDescending(o => o.number)
              .Select(m => m.number)
              .FirstOrDefault() ?? "00001/S2";

the ?? "00001/S2" ?? "00001/S2" might be triggering the error. try doing a break point in var s ... and see if all of s.number has value or has a correct convertible value (there could be just letters which can't be converted)

Answer 2

Expected result:

 item.number // "EMOC-00002/S2" 

You need + "/S2" here:

item.number = "EMOC-" + (++lastPO).ToString("00000") + "/S2";

Explanation

What happens when you ran previous code the second time is that var s = "EMOC-00002" so s.Substring(s.Length - 5 - 3, s.Length - 3) gives "OC-00" instead of "00002" string, which Convert.ToInt32() cannot handle.