Question

I already use linq but search id only and it worked perfectly

var obj = (from VAR in db.infos
           where VAR.id == 22                                                                                
           select new
           {
               title = VAR.title,
           }).SingleOrDefault();
Label2.Text = obj.title.Trim();

If I try to search by location get a error

var obj = (from VAR in db.infos
           where VAR.location.Trim() == "Lim"
           select new
           {
               title = VAR.title.Trim(),
           }).SingleOrDefault();

        SearchJob.Items.Add(obj.title.Trim());
        Label2.Text = obj.title;

Have a error in label2 line

Object reference not set to an instance of an object.

How do I fix it?

if (obj.title != null)
{
    SearchJob.Items.Add(obj.title.Trim());
    Label2.Text = obj.title;
}

Object reference not set to an instance of an object.

SOLUTION

Change SingleOrDefault() to FirstOrDefault()

Answer 1

You’re doing some nasty stuff there, VERY bad habits. For instance this:

var obj = (from VAR in db.infos
           where VAR.location.Trim() == "Lim"
           select new
           {
                title = VAR.title.Trim(),

           }).SingleOrDefault();

SearchJob.Items.Add(obj.title.Trim());
Label2.Text = obj.title;

Is a nonsense! Let me tell you why:

Always check the data BEFORE you insert it into your database, not AFTER. You’re creating a lot of unnecessary overhead this way, which could be avoided altogether. Trim the data before, never after.

Next thing – you need only a single string value, yet you create an anonymous object. WHY? Do this instead:

string title = (from o in db.infos
                where o.location == "Lim"
                select o.title).SingleOrDefault(); 

Use SingleOrDefault if you expect a single result or none. However, if you expect multiple results and want only the first, use FirstOrDefault.

As you can see, I’m using o instead of VAR. It’s true it doesn’t really matter that much, BUT, it’s never a good idea to use a word that’s very similar to one of the reserved words (var).

If you get an exception Object reference not set to an instance of an object., it means that your query returned a null and you’re trying to access a non-existing object. If your query may return null at some point, always check for null when accessing a member!

EDIT

if ( obj.title != null )

is bad too, because you need to check for null the object itself!

if (obj != null)

if you really want to use your bad approach.