Question

I’ve built a custom user control which inherits ComboBox and has items which are

KeyValuePair<bool, string>

I want to set the boolean default value of this ComboBox so that when true it shows “Yes” and when false it shows “No“.

In the code below, when I set the selected value to true, then the ComboBox correctly shows “Yes”.

But when I set the selected value to false, the ComboBox remains blank.

What do I need to do to this user control so that when I set the selected value to false, it shows “No”?

Window1.xaml:

<Window x:Class="TestYesNoComboBox234.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:TestYesNoComboBox234"
    Title="Window1" Height="300" Width="300">
    <StackPanel HorizontalAlignment="Left" Margin="10">
        <StackPanel Orientation="Horizontal">
            <TextBlock Text="Contract available?" Margin="0 0 10 0"/>
            <local:YesNoComboBox Width="60" Height="22" 
               x:Name="ContractAvailable"/>
        </StackPanel>
    </StackPanel>
</Window>

Window1.xaml.cs:

using System.Windows;

namespace TestYesNoComboBox234
{
    public partial class Window1 : Window
    {
        public Window1()
        {
            InitializeComponent();
            //ContractAvailable.SelectedValue = true; //correctly sets "Yes"
            ContractAvailable.SelectedValue = false; //incorrectly does not select anything
        }
    }
}

YesNoComboBox.xaml:

<ComboBox x:Class="TestYesNoComboBox234.YesNoComboBox"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Height="300" Width="300">
    <ComboBox.ItemTemplate>
        <DataTemplate>
            <TextBlock Text="{Binding Value}" />
        </DataTemplate>
    </ComboBox.ItemTemplate>
</ComboBox>

YesNoComboBox.xaml.cs:

using System.Collections.Generic;
using System.Windows;
using System.Windows.Controls;

namespace TestYesNoComboBox234
{
    public partial class YesNoComboBox : ComboBox
    {

        public YesNoComboBox()
        {
            InitializeComponent();
            Loaded += new RoutedEventHandler(YesNoComboBox_Loaded);
        }

        void YesNoComboBox_Loaded(object sender, RoutedEventArgs e)
        {
            SelectedValuePath = "Key";
            Items.Add(new KeyValuePair<bool, string>(true, "Yes"));
            Items.Add(new KeyValuePair<bool, string>(false, "No"));
        }

    }
}

Answer 1

Found the problem. The window constructor happens BEFORE the YesNoComboBox_Loaded. By setting the selected value before the combobox actually has values, for some reason it can only select the first item (try inverting the way you add the KeyValuePairs and set the SelectedValue to false, then try true. False will work, true won’t). I also tried doing it in the Window_Loaded event, but Window_Loaded also happens before YesNoComboBox_Loaded. So what you need to do is add the KeyValuePair items in the constructor of your combobox, and then set the selected value in Window_Loaded (since window constructor ALSO happens before your combobox constructor). Here’s the code:

Window:

public partial class Window1 : Window
{
    public Window1()
    {
        InitializeComponent();
    }

    private void window1_Loaded(object sender, RoutedEventArgs e)
    {
        ContractAvailable.SelectedValue = true;
    }
}

YesNoComboBox:

public partial class YesNoComboBox : ComboBox
{
    public YesNoComboBox()
    {
        InitializeComponent();

        SelectedValuePath = "Key";

        Items.Add(new KeyValuePair<bool, string>(true, "Yes"));
        Items.Add(new KeyValuePair<bool, string>(false, "No"));
    }
}