A simple example of how to make a custom YES/NO dialog with DialogResult in WPF. Freely (as in beer) available code is on Github: https://github.com/Hitman666/CustomYesNoDialogWPF. The breakdown is that you make a new window (MsgBoxYesNo) and assign DialogResult on the button clicks. You call this window as a dialog from a MainWindow and expect a bool in return upon which you act as you wish… MsgBoxYesNo.xaml:

<Window x:Class="CustomYesNoDialogWPF.MsgBoxYesNo"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MsgBoxYesNo" Height="300" Width="300"
        WindowStyle="None" ResizeMode="NoResize"
        WindowStartupLocation="CenterScreen"
        AllowsTransparency="True" Background="Transparent">

    <Border BorderThickness="5" BorderBrush="Black" CornerRadius="20" Background="SkyBlue">
        <Grid>
            <Grid.RowDefinitions>
                <RowDefinition Height="3*"></RowDefinition>
                <RowDefinition Height="1*"></RowDefinition>
            </Grid.RowDefinitions>

            <Grid.ColumnDefinitions>
                <ColumnDefinition></ColumnDefinition>
            </Grid.ColumnDefinitions>

            <Viewbox>
                <TextBlock x:Name="txtMessage" Width="420" FontSize="50" TextWrapping="Wrap" HorizontalAlignment="Center" VerticalAlignment="Center" TextAlignment="Center" Text="Are you sure you're awesome?"/>
            </Viewbox>

            <Viewbox Grid.Row="1">
                <StackPanel Orientation="Horizontal">
                    <Button Content="Yes" x:Name="Yes" Margin="1" HorizontalAlignment="Left" VerticalAlignment="Top" Width="55" Click="Yes_Click"/>
                    <Button Content="No" x:Name="No" Margin="1" HorizontalAlignment="Left" VerticalAlignment="Top" Width="55" Click="No_Click" IsCancel="True"/>
                </StackPanel>
            </Viewbox>
        </Grid>
    </Border>
</Window>

MsgBoxYesNo.cs:

public partial class MsgBoxYesNo : Window
{
    public MsgBoxYesNo(string message)
    {
        InitializeComponent();

        txtMessage.Text = message;
    }

    private void Yes_Click(object sender, RoutedEventArgs e)
    {
        DialogResult = true;
        this.Close();
    }

    private void No_Click(object sender, RoutedEventArgs e)
    {
        DialogResult = false;
        this.Close();
    }
}

MainWindow.cs:

private void Button_Click(object sender, RoutedEventArgs e)
{
    MsgBoxYesNo msgbox = new MsgBoxYesNo("Are you sure you're awesome?");
    if ((bool)msgbox.ShowDialog())
    {
        MessageBox.Show("Yes, you're awesome!");
    }
    else
    {
        MessageBox.Show("You're kidding, of course you're awesome!");
    }
}