MainWindow.xaml

<Window
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:sys="clr-namespace:System;assembly=mscorlib"
        xmlns:dxm="http://schemas.devexpress.com/winfx/2008/xaml/map" 
        x:Class="UseMapItemAttribute.MainWindow"
        xmlns:core="clr-namespace:System;assembly=mscorlib"
        Title="MainWindow" Height="350" Width="525">
    
    <Grid>
        <dxm:MapControl ToolTipEnabled="True" >
            <!--region #AttributeInTooltip-->
            <dxm:VectorLayer Name="itemsLayer">
                <dxm:MapItemStorage>
                    <dxm:MapPolygon  
                        ToolTipPattern="{}Triangle Area: {areaValueAttrName}" 
                        Points="0,0,0,40,40,0,0,0">
                        <dxm:MapPolygon.Attributes>
                            <dxm:MapItemAttribute Name="areaValueAttrName" 
                                                  Type="{x:Type sys:Double}" 
                                                  Value="80" />
                        </dxm:MapPolygon.Attributes>
                    </dxm:MapPolygon>
                </dxm:MapItemStorage>
            </dxm:VectorLayer>
            <!--endregion #AttributeInTooltip-->
        </dxm:MapControl>
    </Grid>
</Window>