WPF C# NULL exception on combobox selection change

This is selection changed event :

private void cbUsers_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    SelectedUser = (sender as ComboBox).SelectedItem.ToString();
    GetUserInformation();
}

GetUserInformation is just selecting password from database. Users are deleted from the database and then the following refreshes the ComboBox items:

public void FillComboBox()
{
    cbUsers.ItemsSource = null;
    HProDataContext db = new HProDataContext();
    var _UserName = (from d in db.users select d.username).ToList();
    cbUsers.ItemsSource = _UserName;
}

HProDataContext db = new HProData开发者_开发百科Context();

var _UserID = (from d in db.users where d.username == cbUsers.Text select d.id).SingleOrDefault();

user u = db.users.Single(p => p.id == _UserID);
db.users.DeleteOnSubmit(u);
db.SubmitChanges();
cbUsers.ItemsSource = null;
cbUsers.Text = null;
FillComboBox();

When using this last method it gives such error:

Object reference not set to an instance of an object.

The error falls on this line of the FillComboBox method:

SelectedUser = (sender as ComboBox).SelectedItem.ToString();

Does anyone have an idea as to what is wrong?

I'd guess that SelectedItem is null and therefore you're calling ToString on nothing.

Consider trying this:

if ((sender as ComboBox).SelectedItem != null)
{
    SelectedUser = (sender as ComboBox).SelectedItem.ToString();
}

However, doesn't your ComboBox have an identifier? This can allow you to refrain from unnecessary conversions with as:

if (myComboBox.SelectedItem != null)
{
    SelectedUser = myComboBox.SelectedItem.ToString();
}

Your event handler is probably getting called when there is no SelectedItem.

I would write this as:

private void cbUsers_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    var usersComboBox = (ComboBox)sender;
    if (usersComboBox.SelectedItem == null) return;

    SelectedUser = usersComboBox.SelectedItem.ToString();
    GetUserInformation();
}

Here you are expecting that sender is always a ComboBox, so I would use a cast instead of as. The exception will let you know if your assumption is wrong.

This will just skip your code if the event handler is called without a selected item.

One possibility is that sender might not be a ComboBox.

A problem of using an as operator without a null check is you get a NullReferenceException instead of a InvalidCastException which is more appropriate

Use an explicit cast instead e.g. ((ComboBox)sender).SelectedItem.ToString(); if you are sure sender is a ComboBox; otherwise check for null before usage.