c# wpf - Count checkbox is checked

after to have read lot of code, I don't find a way to do my idea :

I try to count the number checkbox who are checked.

Here my code xaml :

                <DataGrid Name="dgConnected_Users" AutoGenerateColumns="False" IsReadOnly="True" Height="180" Background="Azure" ScrollViewer.CanContentScroll="True" FontSize="11" BorderBrush="#26BFF0" ScrollViewer.HorizontalScrollBarVisibility="Disabled" HorizontalScrollBarVisibility="Disabled" BorderThickness="2,2,2,2" HeadersVisibility="Column" CanUserAddRows="False" CanUserDeleteRows="False" CanUserResizeRows="False" RowHeight="20" RowBackground="Azure" GridLinesVisibility="Horizontal" HorizontalGridLinesBrush="DarkOrange" SelectionUnit="FullRow" Margin="10,0,10,0">
                <DataGrid.Columns>
                    <DataGridTextColumn FontFamily="Segoe UI Bold" Binding="{Binding UID_UserName}" Header="Utilisateur" Width="1*">
                        <DataGridTextColumn.ElementStyle>
                            <Style TargetType="TextBlock">
                                <Setter Property="VerticalAlignment" Value="Center"></Setter>
                            </Style>
                        </DataGridTextColumn.ElementStyle>
                    </DataGridTextColumn>
                    <DataGridTextColumn Binding="{Binding UID_ComputerName}" Header="Machine" Width="1*">
                        <DataGridTextColumn.ElementStyle>
                            <Style TargetType="TextBlock">
                                <Setter Property="VerticalAlignment" Value="Center"></Setter>
                            </Style>
                        </DataGridTextColumn.ElementStyle>
                    </DataGridTextColumn>
                    <DataGridTemplateColumn Width="24">
                        <DataGridTemplateColumn.CellTemplate>
                        <DataTemplate>
                                <TextBlock TextAlignment="Center" VerticalAlignment="Center"><CheckBox x:Name="Chk_UID_IsSelected"/></TextBlock>
                            </DataTemplate>
                        </DataGridTemplateColumn.CellTemplate>
                        <DataGridTemplateColumn.Header>
                            <Image Source="components/resources/images/delete-24.png" Height="12" Width="12" StretchDirection="Both" Stretch="Fill"/>
                        </DataGridTemplateColumn.Header>
                    </DataGridTemplateColumn>
                </DataGrid.Columns>
            </DataGrid>

and how to obtain the "UID_UserName" of each row with checkbox checked to use in a loop to do a sql request ?