WPF依赖的组合框不会显示.Net 4.5中的SelectedItem

Question

I have main combobox (Categories) and depended combobox (Subcategories). I want it to display SelectedItems when window opens. All works fine in .Net 4.0, but it doesn't work in .Net 4.5. I have two computeres with these .Net versions.

In .net 4.5. only main combobox displays SelectedItem, depended doesn't. How can I fix it?

I made test project to all of you who're interested, just copy and paste. I have no idea how I can make it smaller, sry. But it is simple, clear code example 100% generates the problem.

XAML:

<Window x:Class="GridTest.TestWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:Converter="clr-namespace:GridTest"
        Title="TestWindow" 
        Height="300" 
        Width="300">
    <Window.Resources>
        <Converter:CategoryConverter x:Key="CategoryConverter"/>
    </Window.Resources>
        <Grid>
        <DataGrid Name="_dataGrid" 
                  CanUserAddRows="False" 
                  AutoGenerateColumns="False">
            <DataGrid.Columns>
                <DataGridTemplateColumn Width="*">
                    <DataGridTemplateColumn.CellTemplate>
                        <DataTemplate>
                            <Grid>
                                <Grid.ColumnDefinitions>
                                    <ColumnDefinition/>
                                    <ColumnDefinition/>
                                </Grid.ColumnDefinitions>
                                <ComboBox Grid.Column="0" 
                                          Name="_categories" 
                                          ItemsSource="{Binding Categories}" 
                                          DisplayMemberPath="Name" 
                                          SelectedItem="{Binding SelectedCategory, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}">
                                </ComboBox>
                                <ComboBox Grid.Column="1" 
                                          SelectedItem="{Binding SelectedSubcategory, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" 
                                          DisplayMemberPath="Name">
                                    <ComboBox.ItemsSource>
                                        <MultiBinding Converter="{StaticResource CategoryConverter}">
                                            <Binding Path="Subcategories"/>
                                            <Binding Path="SelectedItem" 
                                                     ElementName="_categories"/>
                                        </MultiBinding>
                                    </ComboBox.ItemsSource>
                                </ComboBox>
                            </Grid>
                        </DataTemplate>
                    </DataGridTemplateColumn.CellTemplate>
                </DataGridTemplateColumn>
            </DataGrid.Columns>
        </DataGrid>
    </Grid>
</Window>

Code:

public class CategoryConverter : IMultiValueConverter
    {
        public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
        {
            if (values[0] == null) return null;

            var subcategories = values[0] as List<Subcategory>;
            if (subcategories == null) return null;

            var category = values[1] as Category;
            if (category == null) return subcategories;

            return subcategories.Where(g => g.CategoryId == category.Id);
        }

        public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }


    public enum CategoryKinds
    {
        Car = 0,
        Fruit = 1,
    }

    public class Category
    {
        public Int32 Id { get; set; }
        public String Name { get; set; }

        public override Boolean Equals(object obj)
        {
            var c = obj as Category;
            if (c == null) return false;

            return Id == c.Id;
        }
    }

    public class Subcategory
    {
        public Int32 Id { get; set; }
        public String Name { get; set; }
        public Int32 CategoryId { get; set; }


        public override Boolean Equals(object obj)
        {
            var sc = obj as Subcategory;
            if (sc == null) return false;

            return Id == sc.Id;
        }
    }

    public class DataGridItem 
    {
        public List<Category> Categories { get; set; }
        public Category SelectedCategory { get; set; }

        public List<Subcategory> Subcategories { get; set; }
        public Subcategory SelectedSubcategory { get; set; }

        public DataGridItem()
        {
            Categories = new List<Category>
            {
                new Category
                {
                    Id = (Int32)CategoryKinds.Car, Name = "Car"
                },
                new Category
                {
                    Id = (Int32)CategoryKinds.Fruit, Name = "Fruit"
                }
            };

            Subcategories = new List<Subcategory>
            {
                new Subcategory
                {
                    Id = 1,
                    Name = "Volvo",
                    CategoryId = (Int32) CategoryKinds.Car
                },
                new Subcategory
                {
                    Id = 2,
                    Name = "Nissan",
                    CategoryId = (Int32) CategoryKinds.Car
                },
                new Subcategory
                {
                    Id = 3,
                    Name = "Banana",
                    CategoryId = (Int32)CategoryKinds.Fruit
                },
                new Subcategory
                {
                    Id = 4,
                    Name = "Lemon",
                    CategoryId = (Int32)CategoryKinds.Fruit
                },
            };
        }
    }

    /// <summary>
    /// Interaction logic for TestWindow.xaml
    /// </summary>
    public partial class TestWindow : Window
    {
        public List<DataGridItem> GridItems { get; set; }

        public TestWindow()
        {
            InitializeComponent();
            DataContext = this;

            GridItems = new List<DataGridItem>
            {
                new DataGridItem
                {
                    SelectedCategory = new Category
                    {
                        Id = (Int32)CategoryKinds.Car, Name = "Car"
                    },
                    SelectedSubcategory = new Subcategory
                    {
                        Id = 2,
                        Name = "Nissan",
                        CategoryId = (Int32) CategoryKinds.Car
                    }
                },
                new DataGridItem
                {
                    SelectedCategory = new Category
                    {
                        Id = (Int32)CategoryKinds.Fruit, Name = "Fruit"
                    },
                    SelectedSubcategory = new Subcategory
                    {
                        Id = 4,
                        Name = "Lemon",
                        CategoryId = (Int32) CategoryKinds.Car
                    }
                }
            };

            _dataGrid.ItemsSource = GridItems;
        }
    }

UPDATE

With approach suggested by Ilan and charly_b code will work fine.

GridItems = new List<DataGridItem>
            {
                new DataGridItem(),
                new DataGridItem()
            };

            GridItems[1].SelectedCategory = GridItems[1].Categories[0];
            GridItems[1].SelectedSubcategory = GridItems[1].Subcategories[1];

            GridItems[0].SelectedCategory = GridItems[0].Categories[1];
            GridItems[0].SelectedSubcategory = GridItems[0].Subcategories[3];

This code will result to:

1. Fruit - Lemon
2. Car - Nissan

But I have solution that will work even if you set SelectedItem that don't belong to ItemsSource of Combobox. You can override GetHashCode method like this:

public override int GetHashCode()
        {
            return Name.GetHashCode();
        }

Obviously, in .Net 4.5 some of WPF methods operating with searching SelectedItem in Combobox's ItemsSource have different implementation from .Net 4.0 and now they use GetHashCode method :)

Answer 1

The Combobox SelectedItem object must be contained inside the Combobox's ItemsSource List.

In order to make your Programm work you can replace the SelectedSubCategory Property with the following code: (I would not use it like this in the production code, but it demonstrates how it works)

    private Subcategory SelectedSubcategoryM;

    public Subcategory SelectedSubcategory
    {
        get
        {
            return this.SelectedSubcategoryM;
        }
        set
        {
            this.SelectedSubcategoryM = (from aTest in this.Subcategories
                                        where aTest.Id == value.Id
                                        select aTest).Single();
        }
    }

Answer 2

Try the next changes, the best practice is to use the source collection items in order to define the selected item. Firstly it is an architectural error to use a new item to define the selection (in both 4.5 and 4 dot.net versions). And second I advice you to use the mvvm approach (including INotifyPropertyChange implementation) to develop wpf related applications, and then all selection logic have to be moved to ViewModel and separated from the code behind (xaml.cs files).

    public MainWindow()
    {
        InitializeComponent();
        DataContext = this;

        var f = new DataGridItem();
        var firstselectedCategory = f.Categories.FirstOrDefault();
        if (firstselectedCategory != null)
        {
            f.SelectedCategory = firstselectedCategory;
            f.SelectedSubcategory =
                f.Subcategories.FirstOrDefault(subcategory => subcategory.CategoryId == firstselectedCategory.Id);
        }
        else
        {

            f.SelectedCategory = null;
            f.SelectedSubcategory = null;
        }
        var s = new DataGridItem();
        var secondSelectedCategory = s.Categories.FirstOrDefault(category => !Equals(category, f.SelectedCategory));

        if (secondSelectedCategory != null)
        {
            s.SelectedCategory = secondSelectedCategory;
            s.SelectedSubcategory =
                s.Subcategories.FirstOrDefault(subcategory => subcategory.CategoryId == secondSelectedCategory.Id);
        }
        else
        {
            s.SelectedCategory = null;
            s.SelectedSubcategory = null;
        }

        GridItems = new List<DataGridItem>
        {
            f,s,
        };

    #region

        //GridItems = new List<DataGridItem>
        //{
        //    new DataGridItem
        //    {
        //        SelectedCategory = new Category
        //        {
        //            Id = (Int32) CategoryKinds.Car,
        //            Name = "Car"
        //        },
        //        SelectedSubcategory = new Subcategory
        //        {
        //            Id = 2,
        //            Name = "Nissan",
        //            CategoryId = (Int32) CategoryKinds.Car
        //        }
        //    },
        //    new DataGridItem
        //    {
        //        SelectedCategory = new Category
        //        {
        //            Id = (Int32) CategoryKinds.Fruit,
        //            Name = "Fruit"
        //        },
        //        SelectedSubcategory = new Subcategory
        //        {
        //            Id = 4,
        //            Name = "Lemon",
        //            CategoryId = (Int32) CategoryKinds.Fruit
        //        }
        //    }
        //};

        #endregion


        _dataGrid.ItemsSource = GridItems;
    }

The xaml code was not changed. How it looks like: . I'll be glad to help if will have problems with the code. Regards.