C# LINQ NULL check | 易学教程

问题

On my form I want to disable the bt_rent button if the movie has no date in the bringback colum(date type) so it means the dvd is still out and not available for renting:

var j = (from s in db.Rentals where s.Movietitle == (string)listbox1.SelectedValue select s).FirstOrDefault();



if (j.Bringback==null)
{
    elerhetotext.Text = "The dvd is not available for renting";
    bt_rent.Enabled = false;
}
else
{
    elerhetotext.Text = "Rentable";
    bt_rent.Enabled = true;
}

The error I get:

Nullreferenceexpection was unhandled by user code (error at:if (j.Bringback==null)) Object reference not set to an instance of an object.

回答1:

You are missing validation on j itself. It seems in this case there is no DVD with the name you're looking for so ideally you should validate if j is NULL as well, instead of

if (j.Bringback==null)

Do the following

if (j != null || (j != null && j.Bringback==null))

So you are checking if the DVD exists, and if it does, the 2nd part of the validation is based on j itself, and if Bringback is null.

回答2:

The Variavle j is a type of object Rental in the given code

<pre>
var j = (from s in db.Rentals where s.Movietitle == (string)listbox1.SelectedValue select s).FirstOrDefault();
</pre>

Chek j is null or not like

<pre>
if (j==null)
{
    elerhetotext.Text = "The dvd is not available for renting";
    bt_rent.Enabled = false;
}
else
{
    elerhetotext.Text = "Rentable";
    bt_rent.Enabled = true;
}
</pre>

回答3:

The exception means that j itself is null, so you would need to check that j is not null before accessing j's members:

if (j == null) {
    elerhetotext.Text = "DVD/movie does not exist";
    bt_rent.Enabled = false;
} else if (j.Bringback == null) {
    elerhetotext.Text = "The dvd is not available for renting";
    bt_rent.Enabled = false;
} else {
    elerhetotext.Text = "Rentable";
    bt_rent.Enabled = true;
}

回答4:

This is only an addition to the accepted answer written by mez

Using C# 6.0's null-conditional operator one could rewrite the code as:

var j = (from s in db.Rentals
         where s.Movietitle == (string)listbox1.SelectedValue
         select s).FirstOrDefault();

// checks whether `j´ is null or `j.Bringback´ is null
if (j?.Bringback == null)
{
    elerhetotext.Text = "The dvd is not available for renting";
    bt_rent.Enabled = false;
}
else
{
    elerhetotext.Text = "Rentable";
    bt_rent.Enabled = true;
}

The statement

if (j != null || (j != null && j.Bringback==null))

would be rewritten as

if (j?.Bringback == null)

来源：https://stackoverflow.com/questions/23193461/c-sharp-linq-null-check

标签

linq

datetime

null

var