WPF - MVVM - C# 在 ItemsSource 更改后无法设置 combobox 的 SelectedItem

Question

I've been struggling with this problem for several days now. I have a combobox that gets its ItemsSource twice. The first time i try to see if i have an offline backup of the data that i am trying to show. While the offline data is showing, the software needs to download the updated data from the database.

Everything works fine if i don't have any stored data in the offline backup. The problem arises when i have a list of items backed up, i put it in the combobox itemsSource, and then change that list after a couple seconds.

The list actually shows correctly, but the selectedItem (which is something that i get from the viewmodel) gets reset and i can't set it again from code-behind. The SelectedItem property in the viewModel is actually properly set, but it doesn't get to the UI.

The list is an ObservableCollection of the same type as the selectedItem.

My combobox

<ComboBox ItemsSource="{Binding SomeList, Mode=OneWay}"
                          SelectedItem="{Binding ListItem, Mode=TwoWay}"
                          DisplayMemberPath="ItemProperty"
                          Margin="10,0,0,0" Width="300" VerticalAlignment="Center"/>  

The way i set the selectedItem in both the offline and online methods:

SomeList= await _mainRepository.BackEndOfflineFirst_GetList();

if (SomeList.Count > 0)
{ 
      ListItem = SomeList.SingleOrDefault(list=> list.Property.Equals(SomeVariable.Property));
}

Any help is appreciated. I should mention that adding IsSynchronizedWithCurrentItem="True" to the combobox always makes the selectedItem the first item of the list.

The check for list.property equals another variable.property ALWAYS returns one single element that is guaranteed to be in the list, and is never null.

Forgot to mention (kinda important, my bad), but i'm using Prism and i implement BindableBase, so the properties are defined as follows:

private ObservableCollection<Type> _someList ;
    public ObservableCollection<Type> SomeList
    {
        get { return _someList ; }
        set { SetProperty(ref _someList , value); }
    }

Answer 1

It is impossible to infer the cause of the problem from your explanations. Everything is correct in the code examples - the problem is somewhere else. Just in case, I ask you to clarify: do you raise PropertyChanged in all properties?

One tip (including as a probable cause), use the ReadOnly property for the observable collection:

public ObservableCollection<...> SomeList {get;}
    = new ObservableCollection<...>();

SomeList.Clear();
foreach(var item in await _mainRepository.BackEndOfflineFirst_GetList())
    SomeList.Add(item);

Answer 2

Found out why it wasn't working.

BindableBase propertyChanged only gets raised when the value actually changes.

I was getting basically the same object (different objects, same value), so it wasn't actually raising the propertyChanged.

To fix this, i put a

ListItem = null;
SomeList.Clear();

inside a finally clause, before actually giving the value i receive from the DB to the ListItem.

It also does not seem to work if i change properties too quickly (hence the finally clause), so i couldn't just put a

SomeList.Clear();
SomeList = await...

back to back.

Another problem i found was that SomeList.Clear() actually throws a NotSupportedException that wasn't actually showing up, hence skipping some of the code.

This was caused by SomeList being an ObservableCollection. Changed it to a generic List and i can Clear it now. If you need to clear the ObservableCollection, a simple Collection = null; works too.

Thank you to everyone that helped me.