If sequence contains no elements show message box

Question

I'm creating a log in form in c# using this code

DataclassesDatacontext db = new DataclassesDatacontext();
Student s = null

s = db.Students.Single(r => r.Username == textboxUname.Text && r.Password == textboxPword.Text);
if (s != null)
{
  form1 frm = new form1();
  frm.Show();
}

if the user inputs a valid account, form1 will show but if the user inputs invalid account or the username and password he entered does not exist in the database, i'll get an error on the third line saying "sequence contains no elements".

Now my problem is, how to show a message box if the sequence contains no elements without using try catch.

Thanks in advance

else? if s does not equal null, show the form, otherwise, show a messagebox.. else { Messagebox.show("xxx") } Yeah, also the singleordefault others have stated. I didn't scroll all the way to the right, assumed if you were doing a null check it was already there.. mb — Aaron, Sep 23 '16 at 15:02

score 3 · Accepted Answer · answered Sep 23 '16 at 15:02

3

I assume that Single is the cause of the exception. You should use SingleOrDefault instead.

s = db.Students.SingleOrDefault(r => r.Username == textboxUname.Text && r.Password == textboxPword.Text);

When the collection is empty, SingleOrDefault shall return null instead of throwing an exception like the function Single.

answered Sep 23 '16 at 15:02

Maor Veitsman

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Wow! Never thought this could be so easy. Thank you so much. It works great! – Leir Sep 23 '16 at 15:54

eavidan · Answer 2 · 2016-09-23T15:06:35.520

-1

As Aaron suggested, an else block will suffice in this case, if you use SingleOrDefault instead. In general, you can use the Any() function to test if any element exists (in your case, combined with a Where())

edited Sep 23 '16 at 15:06

answered Sep 23 '16 at 15:04

eavidan

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That´s not true, because `Single` will *never* return null and thus the `else` is not reachable at all. Anyway this is merely a comment than an answer. – MakePeaceGreatAgain Sep 23 '16 at 15:05
You can also use `Any` together with a filter-crtierion, no need for a `Where`. – MakePeaceGreatAgain Sep 23 '16 at 15:07

If sequence contains no elements show message box

2 Answers2