ComboBox- SelectionChanged event has old value, not new value

The correct value to check here is the SelectedItem property.

A ComboBox is a composite control with two of its parts being:

1. The Text Part: the value in the this part corresponds to the Text property of the ComboBox.
2. The Selector Part (i.e. the "drop-down" part): The selected item in this part corresponds to the SelectedItem property.

The image above was taken immediately after the ComboBox was expanded (i.e. before selecting a new value). At this point both Text and SelectedItem are "Info", assuming the ComboBox items were strings. If the ComboBox items were instead all the values of an Enum called "LogLevel", SelectedItem would currently be LogLevel.Info.

When an item in the drop-down is clicked on, the value of SelectedItem is changed and the SelectionChanged event is raised. The Text property isn't updated yet, though, as the Text Part isn't updated until after the SelectionChanged handler is finished. This can be observed by putting a breakpoint in the handler and looking at the control:

Since the Text Part hasn't been updated at this point, the Text property returns the previously selected value.

Use the DropDownClosed event instead of selectionChanged if you want the current value of the combo box.

private void comboBox_DropDownClosed(object sender, EventArgs e)
{
   MessageBox.Show(comboBox.Text) 
}

Is really that simple.

This worked for me:

private void AppName_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
   ComboBoxItem cbi = (ComboBoxItem)AppName.SelectedItem;
   string selectedText = cbi.Content.ToString();
}

According to MSDN, e.AddedItems:

Gets a list that contains the items that were selected.

So you could use:

private void OnMyComboBoxChanged(object sender, SelectionChangedEventArgs e)
{
    string text = (e.AddedItems[0] as ComboBoxItem).Content as string;
}

You could also use SelectedItem if you use string values for the Items from the sender:

private void OnMyComboBoxChanged(object sender, SelectionChangedEventArgs e)
{
    string text = (sender as ComboBox).SelectedItem as string;
}

or

private void OnMyComboBoxChanged(object sender, SelectionChangedEventArgs e)
{
    string text = ((sender as ComboBox).SelectedItem as ComboBoxItem).Content as string;
}

Since both Content and SelectedItem are objects, a safer approach would be to use .ToString() instead of as string