Wpf combobox selected item not working

Question

I have two object: UserDto and RoleDto. User has a property which is the RoleDto. In my viewmodel I have the following:

public UserDto User
    {
        get { return _user; }
        set
        {
            if (_user == value) return;

            _user = value;
            User.PropertyChanged += UserPropertyChanged;
            OnPropertyChanged("User");
        }
    }
    private UserDto _user;

public IEnumerable<RoleDto> Roles { get; set; } //I load all available roles in here

In the view, I want to select the role that the user belongs. This is how I define the combobox in the view:

<ComboBox Grid.Row="3" Grid.Column="1" Margin="5" ItemsSource="{Binding Roles}" SelectedItem="{Binding User.Role, Mode=TwoWay, ValidatesOnDataErrors=True, UpdateSourceTrigger=PropertyChanged}" DisplayMemberPath="Description" />

If I try to create a new user and select a role from the combobox, it is correctly binded to the user. The problem is that when I load a user that already exists, the role is not displayed in the combobox (even the user has a role defined).

Any help please?

Thanks in advance

Solution 1

This is because the reference of RoleDTO that your UserDTO has, does not match any of the RoleDTOs in Roles collection which you set as ItemsSource of ComboBox.

Better define a property on your ViewModel like

    public RoleDTO SelectedRole
    {
        get { return Roles.FirstOrDefault(role => role.Role == User.RoleDto.Role); }
        set { User.RoleDto = value; OnPropertyChanged("SelectedRole"); }
    }

and set it as SelectedItem of you combobox

ItemsSource="{Binding Roles}" SelectedItem="{Binding SelectedRole, Mode=TwoWay, ValidatesOnDataErrors=True, UpdateSourceTrigger=PropertyChanged}" DisplayMemberPath="Description" />

Solution 2

In my opinion the second option on this page is the easiest way.

https://rachel53461.wordpress.com/2011/08/20/comboboxs-selecteditem-not-displaying/

You can override the equals property on your object so that it returns true if the items have the same data. Then when the combo box box goes to check to make sure your item is in the selection it will find a match.