I have a datagrid in WPF that I am binding to an object.

I have a DataGridCheckBoxColumn on there which I want the users to be able to go through and tick the ones they want. Problem is they have to click twice, once for selection then again to check/uncheck. How on earth do you turn this off, I've been searching for way to long to find the answer to this. The datagrid has SelectionMode and SelectionUnit properties - neither of which accept 'none' or 'go away'

Any help is appreciated! My code is below for reference

<my:DataGrid Margin="15"  Name="dgPreview" 
                    AutoGenerateColumns="False" CanUserSortColumns="True" 
                         CanUserDeleteRows="True" 
                         Background="White" 
                         ColumnHeaderHeight="20" 
                         VerticalScrollBarVisibility="Visible" 
                         RowDetailsVisibilityMode="Visible" 
                         >

                <my:DataGrid.Columns>
                    <my:DataGridCheckBoxColumn  MinWidth="50" Width="Auto" Header="Include" Binding="{Binding Include}" />
                    <my:DataGridTextColumn MinWidth="50"  Width="Auto" Header="Override #" Binding="{Binding OverrideNumber}" />
                    <my:DataGridTextColumn MinWidth="220" Width="*" Header="Name" Binding="{Binding Name}" />
                    <my:DataGridTextColumn MinWidth="50" Width="Auto" IsReadOnly="True"  Header="Preview" Binding="{Binding Preview}" />
                </my:DataGrid.Columns>
            </my:DataGrid>