Question

I’m having troubles creating a custom control with a ComboBox.
This is my simple code:

public class MyComboBox : Control
{
    public IEnumerable ItemsSource
    {
        get { return (IEnumerable)GetValue(ItemsSourceProperty); }
        set { SetValue(ItemsSourceProperty, value); }
    }

    // Using a DependencyProperty as the backing store for ItemsSource.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty ItemsSourceProperty =
        DependencyProperty.Register("ItemsSource", typeof(IEnumerable), typeof(MyComboBox), new UIPropertyMetadata(null));




    public string DisplayMemberPath
    {
        get { return (string)GetValue(DisplayMemberPathProperty); }
        set { SetValue(DisplayMemberPathProperty, value); }
    }

    // Using a DependencyProperty as the backing store for DisplayMemberPath.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty DisplayMemberPathProperty =
        DependencyProperty.Register("DisplayMemberPath", typeof(string), typeof(MyComboBox), new UIPropertyMetadata(""));





    public string SelectedValuePath
    {
        get { return (string)GetValue(SelectedValuePathProperty); }
        set { SetValue(SelectedValuePathProperty, value); }
    }

    // Using a DependencyProperty as the backing store for SelectedValuePath.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty SelectedValuePathProperty =
        DependencyProperty.Register("SelectedValuePath", typeof(string), typeof(MyComboBox), new UIPropertyMetadata(""));





    public object SelectedValue
    {
        get { return (object)GetValue(SelectedValueProperty); }
        set { SetValue(SelectedValueProperty, value); }
    }

    // Using a DependencyProperty as the backing store for SelectedValue.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty SelectedValueProperty =
        DependencyProperty.Register("SelectedValue", typeof(object), typeof(MyComboBox), new UIPropertyMetadata(null));




    public int SelectedIndex
    {
        get { return (int)GetValue(SelectedIndexProperty); }
        set { SetValue(SelectedIndexProperty, value); }
    }

    // Using a DependencyProperty as the backing store for SelectedIndex.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty SelectedIndexProperty =
        DependencyProperty.Register("SelectedIndex", typeof(int), typeof(MyComboBox), new UIPropertyMetadata(0));



    static MyComboBox()
    {
        DefaultStyleKeyProperty.OverrideMetadata(typeof(MyComboBox), new FrameworkPropertyMetadata(typeof(MyComboBox)));
    }
}

and this is its Generic.xaml:

<Style TargetType="{x:Type local:MyComboBox}">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type local:MyComboBox}">
                <Grid>
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition />
                        <ColumnDefinition />
                    </Grid.ColumnDefinitions>
                    <TextBlock Grid.Column="0" Text="MyComboBox" />
                    <ComboBox Grid.Column="1"
                              ItemsSource="{Binding Path=ItemsSource, RelativeSource={RelativeSource Mode=TemplatedParent}}"
                              SelectedIndex="{Binding Path=SelectedIndex, RelativeSource={RelativeSource Mode=TemplatedParent}}"
                              DisplayMemberPath="{Binding Path=DisplayMemberPath, RelativeSource={RelativeSource Mode=TemplatedParent}}"
                              SelectedValuePath="{Binding Path=SelectedValuePath, RelativeSource={RelativeSource Mode=TemplatedParent}}"
                              SelectedValue="{Binding Path=SelectedValue, RelativeSource={RelativeSource Mode=TemplatedParent}}">
                    </ComboBox>
                </Grid>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

To test it, I made a simple WPF application with this MainWindow.xaml:

<Window x:Class="Example.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:Example"
        Title="MainWindow" Height="350" Width="525">
    <Window.DataContext>
        <local:ViewModel />
    </Window.DataContext>
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition />
            <ColumnDefinition />
        </Grid.ColumnDefinitions>

        <ComboBox Grid.Row="0" Grid.Column="0" Margin="4" VerticalAlignment="Center"
                  ItemsSource="{Binding Path=Numbers}"
                  DisplayMemberPath="Key"
                  SelectedValuePath="Value"
                  SelectedValue="{Binding Path=Number, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
        <TextBlock Grid.Row="0" Grid.Column="1" Margin="4" HorizontalAlignment="Center" VerticalAlignment="Center"
                   Text="{Binding Path=Number, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />

        <local:MyComboBox Grid.Row="1" Grid.Column="0" Margin="4" VerticalAlignment="Center"
                          ItemsSource="{Binding Path=MyNumbers}"
                          DisplayMemberPath="Key"
                          SelectedValuePath="Value"
                          SelectedValue="{Binding Path=MyNumber, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
        <TextBlock Grid.Row="1" Grid.Column="1" Margin="4" HorizontalAlignment="Center" VerticalAlignment="Center"
                   Text="{Binding Path=MyNumber, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
    </Grid>
</Window>

and this ViewModel:

public class ViewModel : INotifyPropertyChanged
{
    private int _number;
    public int Number
    {
        get { return _number; }
        set
        {
            _number = value;
            OnPropertyChanged("Number");
        }
    }

    public Dictionary<string, int> Numbers { get; set; }

    private int _myNumber;
    public int MyNumber
    {
        get { return _myNumber; }
        set
        {
            _myNumber = value;
            OnPropertyChanged("MyNumber");
        }
    }

    public Dictionary<string, int> MyNumbers { get; set; }

    public ViewModel()
    {
        Numbers = new Dictionary<string, int>()
        {
            { "One", 1 },
            { "Two", 2 },
            { "Three", 3 }
        };
        Number = 1;

        MyNumbers = new Dictionary<string, int>()
        {
            { "Four", 4 },
            { "Five", 5 },
            { "Six", 6 }
        };
        MyNumber = 4;
    }

    public event PropertyChangedEventHandler PropertyChanged;
    private void OnPropertyChanged(string name)
    {
        PropertyChangedEventHandler e = PropertyChanged;
        if (e != null)
        {
            e(this, new PropertyChangedEventArgs(name));
        }
    }
}

When I start it, my custom control has a red border and the output Window of Visual Studio signal this error:

System.Windows.Data Error: 23 : Cannot convert '[Four, 4]' from type 'KeyValuePair`2' to type 'System.Int32' for 'en-US' culture with default conversions; consider using Converter property of Binding. NotSupportedException:'System.NotSupportedException: Int32Converter cannot convert from System.Collections.Generic.KeyValuePair`2[[System.String, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[System.Int32, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].
   at System.ComponentModel.TypeConverter.GetConvertFromException(Object value)
   at System.ComponentModel.TypeConverter.ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, Object value)
   at System.ComponentModel.BaseNumberConverter.ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, Object value)
   at MS.Internal.Data.DefaultValueConverter.ConvertHelper(Object o, Type destinationType, DependencyObject targetElement, CultureInfo culture, Boolean isForward)'
System.Windows.Data Error: 7 : ConvertBack cannot convert value '[Four, 4]' (type 'KeyValuePair`2'). BindingExpression:Path=MyNumber; DataItem='ViewModel' (HashCode=55591935); target element is 'MyComboBox' (Name=''); target property is 'SelectedValue' (type 'Object') NotSupportedException:'System.NotSupportedException: Int32Converter cannot convert from System.Collections.Generic.KeyValuePair`2[[System.String, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[System.Int32, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].
   at MS.Internal.Data.DefaultValueConverter.ConvertHelper(Object o, Type destinationType, DependencyObject targetElement, CultureInfo culture, Boolean isForward)
   at MS.Internal.Data.ObjectTargetConverter.ConvertBack(Object o, Type type, Object parameter, CultureInfo culture)
   at System.Windows.Data.BindingExpression.ConvertBackHelper(IValueConverter converter, Object value, Type sourceType, Object parameter, CultureInfo culture)'

The problem disappear when I remove the property SelectedIndex from the Generic.xaml, but I need it because I want that who will use my control, can have the same basic functionality of a ComboBox.
Anyone knows how to solve it?

Answer 1

I find the solution by myself.
The problem is on the default value I inserted for SelectedIndex: it must be -1 and not 0.