Must have non-null value for 'Setter.Property'

Question

I am reading this article on MSDN site, in order to understand DataTrigger.

I have created a DefaultViewModel class which looks like this.

        namespace ControlTemplateDemo
{
    public class DefaultViewModel
    {
        private List<ToDoItem> _list;

        public DefaultViewModel()
        {
            _list = new List<ToDoItem>();
            _list.Add(new ToDoItem { TaskName="Wedding",Priority = 1,Description="Important wedding",TypeOfTask = TaskType.Home});
            _list.Add(new ToDoItem { TaskName = "Toyota Meeting", Priority = 3, Description = "WSR", TypeOfTask = TaskType.Work });
        }

        public List<ToDoItem> Tasks
        {
            get { return _list; }
        }
    }
}

and my xamal code look like this.

<Window x:Class="ControlTemplateDemo.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:local="clr-namespace:ControlTemplateDemo"
    xmlns:sys="clr-namespace:System;assembly=mscorlib"
    Title="MainWindow" Height="350" Width="525">
<Window.Resources>

    <DataTemplate  DataType="{x:Type local:ToDoItem}">
        <Border BorderThickness="1"  Name="myBorder" Margin="5" Padding="5" BorderBrush="Aqua">
            <Grid>
                <Grid.RowDefinitions>
                    <RowDefinition/>
                    <RowDefinition/>
                    <RowDefinition/>
                </Grid.RowDefinitions>
                <Grid.ColumnDefinitions>
                    <ColumnDefinition/>
                    <ColumnDefinition/>
                </Grid.ColumnDefinitions>
                <TextBlock Text="Task Name" Grid.Row="0" Grid.Column="0"/>
                <TextBlock Text="{Binding TaskName}" Grid.Row="0" Grid.Column="1"/>

                <TextBlock Text="Description:" Grid.Row="1" Grid.Column="0"/>
                <TextBlock Text="{Binding Description}" Grid.Row="1" Grid.Column="1" />

                <TextBlock Text="Priority:" Grid.Row="2" Grid.Column="0"/>
                <TextBlock Text="{Binding Priority}"  Grid.Row="2" Grid.Column="1"/>                                        
            </Grid>
        </Border>
        <DataTemplate.Triggers>
            <DataTrigger Binding="{Binding Path=TypeOfTask}">
                <DataTrigger.Value>
                    <local:TaskType>Home</local:TaskType>
                </DataTrigger.Value>
                <Setter TargetName="myBorder" Property="Background" Value="Yellow" />
            </DataTrigger>

            <MultiDataTrigger>
                <MultiDataTrigger.Conditions>
                    <Condition Binding="{Binding Path=Priority}">
                        <Condition.Value>
                            <sys:Int32>3</sys:Int32>
                        </Condition.Value>
                    </Condition>
                    <Condition Binding="{Binding Path=Description}">
                        <Condition.Value>
                            <sys:String>WSR</sys:String>
                        </Condition.Value>
                    </Condition>
                </MultiDataTrigger.Conditions>
                <Setter>
                    <Setter.TargetName>myBorder</Setter.TargetName>
                    <Setter.Property>Background</Setter.Property>
                    <Setter.Value>Green</Setter.Value>
                </Setter>
            </MultiDataTrigger>

        </DataTemplate.Triggers>

    </DataTemplate>

    <!--<local:DefaultViewModel x:Key="dvm"/>-->
</Window.Resources>
<Grid>
    <StackPanel>
        <TextBlock Name="blah" FontSize="20" Text="My To do tasks."/>
        <ListBox x:Name="lstTasks" ItemsSource="{Binding Path=Tasks}" HorizontalContentAlignment="Stretch" >
        </ListBox>
    </StackPanel>
</Grid>

Qeustion/Problem:
I am using a DataTemplate to reder data inside ListBox. My intention is to highlight background of border ( border of list item ) depending on multiple conditions. e.g. if TaskPriorty is 3 and TaskDescription is 'WSR' then I want to highlight that list item with red color. However when I am running this application I am getting runtime error with below message. Must have non-null value for 'Setter.Property'.

Can anyone help me to find out the problem? Without MultiDataTrigger everything is working fine.

Thanks, Hemant

Answer 1

Use attribute syntax instead of property element syntax

<MultiDataTrigger>
    <MultiDataTrigger.Conditions>
    ...
    </MultiDataTrigger.Conditions>
    <Setter TargetName="myBorder" Property="Background" Value="Green"/>
</MultiDataTrigger>